RBBS in a Box Volume 1 #3.1

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Notes and Observations - on - IBM PC-DOS and Microsoft MS-DOS - Releases 2.0 and 2.1 - April, 1984 This document is a collection of data from various sources, including bulletin boards, magazines, seminar handouts, and independent research. WARNING: This information is provided without warranty of any kind. Each individual must determine the applicability and accuracy ofthis information to their specific environment. If you have corrections, additions, or comments, please send them to: John Chapman 844 S. Madison St. Hinsdale, Illinois 60521 Or, send EMAIL to userid [70205,1217] on CompuServ, or MAIL to userid $AC on PCSHARE BATCH PROCESSING To modify the DOS 2.0 command interpreter so that the default is echo off when a batch file is run, C>debug \command.com -e1721 28 1f -e364a 24 26 c6 06 6e 09 00 e9 19 e8 -w Writing 4500 bytes -q The DOS Environment is accessible for use in BATs. The standard keywords COMSPEC, PATH, PROMPT et al can all be read and set. New keywords can be set and checked within BATs. The keywords are saved across BAT executions. For instance BATs can check which screen is active using SET conventions as follows: . NOTE leading and trailing Percent signs IF %SCREEN% == MONO GOTO COLOR IF %SCREEN% == COLOR GOTO MONO :COLOR SET SCREEN=COLOR . NOTE no spaces around '=' in SET MODE CO80 EXIT :MONO SET SCREEN=MONO MODE MONO EXIT . SCREEN is set and saved across BAT executions Another example using a link convention allows BATs to call BATs: *****CALL.BAT***** IF NOT %BATLABEL% == . GOTO %BATLABEL% . note BATLABEL is assumed to be set to '.' as a null value ECHO DO SOME STUFF HERE SET BATCALR=%0 . give the called BAT my name to get back . then leave word where to come back to SET BATLABEL=RETURN . set further parms to save %1 thru %9 if necessary CALLED :RETURN ECHO FINISH UP HERE AFTER RETURN PC/MS-DOS Tips (2.0/2.1) 2 The DOS Environment is accessible for use in BATs. (Cont'd) *****CALLED.BAT***** ECHO DO STUFF WE WERE CALLED TO DO . return to caller %BATCALR% . separate keywords will be necessary if further calls . will be made A note of caution (this is documented), the environment at boot time, is limited (I believe to only 128 bytes). If any resident modules are installed (i.e. PRINT, MODE, or GRAPHICS) the area cannot be expanded. You can SET enough variables from within AUTOEXEC.BAT to get enough space to get by. I would also recommend setting up a set of conventions and preinitializing the parms you'll use to '.'; Null parameters are removed from the environment. In summary - The rules are simple SET a parm without percent signs refer to them by enclosing them in percent signs I hope this discovery will generate a lot of re-thinking some BAT techniques. PC/MS-DOS Tips (2.0/2.1) 3 Current MSDOS 1.xx - 2.xx Disk formats (T. Jennings 19 Aug 83) Disk Type Type Code ....................................................... Single Density Single Sided 8" (SD128) **** Double Density Single Sided 8" (DD1024) **** FIDO's 8" Double Density (DD1K) Double Density Double Sided 8" (DD1024-2) IBM Displaywriter System disk (SD256) IBM Displaywriter System disk (DD256-2) IBM PC 8 Sector Single Sided (IBM8) IBM PC 9 Sector Single Sided (IBM9) IBM PC 8 Sector Double Sided (IBM8-2) IBM PC 9 Sector Double Sided (IBM9-2) Single Density Double Sided 8" (SD128-2) **** Never did get Microsoft to figure out which one these is the "correct" 8 inch format. Who knows. Type Dir Disk Fats Blk Res Sec FAT Code Size Size size secs size ID ..................................................... SD128 68 251K 2 512 1 128 FE DD1K 192 660K 2 1024 1 1024 FE DD1024 96 612K 2 1024 1 1024 FE DD1024-2 192 1232K 2 1024 1 1024 FF SD256 80 287K 2 512 2 256 FA Note 1 DD256-2 172 1001K 2 1024 2 256 FB Note 2 IBM8 64 162K 2 512 1 512 FE IBM9 64 180K 2 512 1 512 FC IBM8-2 112 320K 2 1024 1 512 FF IBM9-2 112 360K 2 1024 1 512 FD SD128-2 68 2 512 4 128 FC Type trks secs res sec FAT dir 1st 2nd 1st 1st totl num. Code trk secs size size size FAT FAT dir data secs heads ...................................................................... SD128 77 26 1 128 6 17 1 7 13 30 2002 1 DD1024 77 8 1 1024 1 3 1 2 3 6 616 1 DD1024-2 77 8 1 1024 2 6 1 3 5 11 1232 2 SD256 77 15 17 256 4 10 2 6 10 20 1155 1 Note 1 DD256-2 77 26 54 256 6 20 2 8 14 34 4004 2 Note 2 IBM8 40 8 1 512 1 4 1 2 3 7 320 1 IBM9 40 9 1 512 2 4 1 3 5 9 360 1 IBM8-2 40 8 1 512 1 7 1 3 5 10 640 2 IBM9-2 40 9 1 512 2 7 1 3 5 12 720 2 SD128-2 77 26 4 128 12 17 4 16 28 45 4004 2 Note 1: 15 sector bias in BIOS Note 2: 52 sector bias in BIOS PC/MS-DOS Tips (2.0/2.1) 4 Additional feature in DEBUG command There is a compare feature in DEBUG that is not in the documentation. Two blocks of memory can be compared, byte by byte with the following command: -C AD1 AD2 AD3 DEBUG will compare the memory block from AD1 to AD2 with the memory block starting at AD3. The results appear in a table showing both blocks side by side with the addresses and the corresponding contents. (from IBM.PC Users Group of Winnipeg) PC/MS-DOS Tips (2.0/2.1) 5 Program File Formats The order of the segments in memory is determined by the linker; and all the segments are contiguous, unless you do some really determined and tricky hacking. Supposedly, the linker determines the order of the segments by putting them in "alphabetical order by segment name", or at least that's what the documentation says it's supposed to do; but I've found that it puts them in the order in which they're encountered in the program source code, unless you create a dummy file (see LINK instructions) with a different sequence and make it first in the LINK list of OBJ modules. (I generally put them in source in the sequence code, data, stack; they always come out in that order, regardless of segment names, unless I use the dummy file. Let's assume that sequence for the remainder of this discussion.) With an .EXE file, just before execution, the registers are as follows: AX - ostensibly per doc, zero; seems to contain the number of characters in the command tail, though BX:CX - 32-bit number showing load module memory size in bytes (good for dynamically allocating memory) DX - zero SS:SP - if you defined a stack segment, these are loaded accordingly; if not, SS=CS, SP=0FFFFH or end of memory DS and ES - SEGMENT address of "program header" (see below) CS:IP - far address of label in "END" statement of program The program header is the standard 256-byte .COM file "header", similar to what you would expect to see at 00-FFH in CP/M 2.x (but see doc for important differences!). Therefore, DS = CS - 10H (which is 100H divided by 10H) if the code seg is the first (owest) in memory, because it points to a location 100H before the start of the code seg. PC/MS-DOS Tips (2.0/2.1) 6 Program File Formats (Cont'd) Now, if CS is, say, 9E3H when you come up, then DS and ES will contain 9D3H. (Remember, though, that your code thinks it starts at 9E3:0, zero being the address you specified in your END directive, and the instruction pointer will actually contain zero.) These values have nothing at all to do with the place at which your data seg is located; they point at that "program header". At location 9D3:80 (DS:80) is the command tail, if you're gonna reference that; its length is in AX (I believe) and is also pointed to by the expression DS:80 (just like CP/M; data starts at 81H), or to say it another way, in this case absolute memory location 09DB0 (09D30+80). But your code segment is, say, 400H long, and the linker knew that 'cause the assembler told it so in the seg, the instructions MOV AX,DATASEG MOV DS,AX MOV ES,AX (where DATASEG is the name of your data segment) will have been resolved by the linker in this way; the first instruction of those three was interpreted by the assembler as an immediate move, but the "data" to be moved into AX was marked as relocatable. The linker leaves instructions for the loader to add 9E3H (or whatever address the loader decided was a good place to put the program) to the value of the "constant" DATASEG, which is equal to 40H (400H/10H) because it's being "loaded" into a segment register, and when you look at that instruction under DEBUG it would say MOV AX,0A23H. (Obviously, this is the process of address resolution.) In absolute addresses, the codeseg starts at 09E30H, and the dataseg 400H later at 0A230H; in 8086 shorthand that's 9E3:0 and A23:0 respectively. (This assumes the stackseg is last, say at A98:0; that would imply that your data seg was (0A98H-0A23H)*10H, or 750H, in length.) Almost all instructions accessing data use DS as "base" register, unless you override that with a prefix (as in MOV AX,ES:BLKCTR). All instructions accessing storage require one of the segregs as "base" register. (String primitives ALWAYS work with DS and/or ES and can't be overridden by a prefix; instructions using BP as an "index" are relative to SS, unless overridden by a prefix; otherwise, it's DS when not overridden. Be careful!) PC/MS-DOS Tips (2.0/2.1) 7 Program File Formats (Cont'd) In BAL, you give the assembler a USING statement; that tells it that offsets will be computed relative to the first operand of the USING statement, and tells it to use the second operand as "base" register in base+offset instructions. The code won't work properly until you load the address of the first operand into the register named by the second operand. In 8086, you give the assembler an ASSUME statement; for instructions using CS as "base" (segment) register (all of them), such offsets will be calculated from the point named as xyz in the statement "ASSUME CS:xyz, DS:abc,ES:def"; for instructions which use DS (such as MOV AX,BLKCTR), the value of the offset of BLKCTR is computed relative to label abc; for ES instructions (when you use a segment override prefix, as in MOV AX,ES:BLKCTR), offset is computed relative to "def". Then, if you hav that (for example) DS:offset points to the absolute location of BLKCTR, you'll be moving something random into your AX register. With .COM files, CS=DS=ES at startup; note that a typical .COM file ASSUME statement looks like ASSUME CS:CODESEG,DS:CODESEG,ES:CODESEG,SS:CODESEG (whoops, one more) But as you can see from the preceding stuff, with .EXE files DS and ES start out with values which don't point at your data at all. You have to do something like MOV AX,abc!MOV DS,AX!MOV ES,AX to put coherent values into the segregs, just as you have to do something like BALR 4,0!BCTR 4,0!BCTR 4,0 to put a coherent value into the base reg in BAL. That is, with an .EXE file, you tell the assembler to use DATASEG as the reference point to compute an offset from (with the ASSUME statement); and then you must make sure you load DS with a value that will, in combination with the computed offset, point at the right data. One more example, then I'll stop. Let's take a BAL program and specify a code "segment" 4096 bytes long, a data "segment" 4096 bytes long, and a "stack segment" after that. In order to be able to access all of that data in that way, you'd need three base registers; let's use 3, 4 and 5. With BAL, you have to figure the addresses yourself, so we get something like: PGM START 0 BEGIN STM 14,12,12(13) BALR 3,0 DTSEG EQU START+4096 STSEG EQU START+8192 USING *,3 USING DTSEG,4 USING STSEG,5 START ST 13,SAVEAREA+4 LA 13,SAVEAREA LA 4,4096(3) LA 5,8192(3) That is, if I remember my BAL. You get the idea. PC/MS-DOS Tips (2.0/2.1) 8 The CTTY Command and Basic Feature (from Tom Jennings) The DOS manual explains that the command is simply CTTY device-name. This will transfer control from the keyboard and screen to that device. It suggests that LPT1: is not a ood idea since the printer communicates (mostly) one way. What this really leaves is COM1:, COM2: and AUX: I very cleverly tried typing CTTY COM1: It does do something, it kills the keyboard and a cold BOOT is necessary to restart. Well, that wasn't it. Something must occur prior to the CTTY command. Being strickly a seat-of-the-pants type, I set the HAYES to ANSWER and called in. Nice, I had a connection (CARRIER/carrier). I rushed over to the PC keyboard and typed CTTY COM1: and the screen went dead. So, what else is new....But, on my other SYSTEM, (other end of the office), the prompt A> appeared. I sat and typed on the terminal just as though I was sitting in front of the IBM. Well, what do you know? 1) Commands are limited to what the terminal keyboard can reproduce i.e. ^C for CTRL-BREAK but not CTRL-ALT-DEL (as if you would want that one anyway). 2) Going to BASIC cuts you off. SYSTEM control returns to the keyboard. 3) Most commands work except COPY as it relates to CON: or COM1: COPY from disk to disk worked O.K. 4) This means is total control from outside while in DOS only on a line-by-line basis. Anything which relies on screen mapping will not work. (Run Norton's DISKMAP from the terminal and the map appears on the PC screen.) You may be wondering why all this verbage. Well.... the result of my efforts is a BASIC program called RING.BAS which will set up the HAYES to patiently look for an incoming ring, and very politely turn SYSTEM control over to the caller. Oh, I forgot to mention the BASIC program must be called from a BATCH file called OUTSIDER.BAT. When RING goes back to the SYSTEM, the next command is CTTY COM1: Slick, huh? RING will even write the BATCH file for you. Just take a look at the internal documentation for additional information. Note: Tom Jenning's 'FIDO' board is at (415) 864-1418 PC/MS-DOS Tips (2.0/2.1) 9 DOS 2.0 HAS PROBLEMS WITH REDIRECTION OF I/O There are problems in DOS 2.0 with the redirection of I/O and piping for programs that use the original DOS 1.1 INT 21 function calls for input. This problem is readily apparent to users of C language packages such as Computer Innovation C-86, Lattice C, or Microsoft C (you'd think they would get it right!). One problem is that all output to the screen is redirected, even keyboard echo. Correct operation would redirect all program output for the screen (stdout) to the specified >file, but the echo of keyboard input would still be sent to the screen. Instead, both the keyboard echo and the program output are sent to the redirected >file. Thus, if you run programs such as the CAT.C (K&R,page 154) example that Microsoft distributes with their C; or COPYIO.C (K&R,page 15) with the output redirected to a file, you will get the following results: 1. Under DOS 1.1, keyboard input is echoed to the screen as you type and each line appears in the >file once as expected. 2. Under DOS 2.0, keyboard input is not echoed to the screen, but each line appears in the >file twice! This situation is handled correctly in DOS 2.0 if the new INT 21 function call 3F is used. This can be demonstrated by redirecting output for the DOS 2.0 function MORE - it works as desired. The redirecting of input to these programs doesn't work properly either. If the file has not been edited with debug to end with a control-Z, the program will hang up at the end of the <input file. You must reboot the system to continue! Also, if you pipe the output of the first program into a second program, the final output will contain each line four times, doubled spaced after the second lie! These problems do not occur for programs that use the new DOS 2.0 calls for I/O, such as SORT and MORE. The question now is how do you fixup C programs to run under DOS 2.0 and not redirect keyboard echo to the stdout file? The easiest way for C compilers that include their own redirection code is to change their redirection symbols from <, >, and >> to something else. Then DOS 2.0 won't do the redirection, so the C code will be able to do it correctly. With the Microsoft C compiler, this is easily accomplished by modifying three lines of code in MAIN.C. A good choice is to modify MAIN.C so that it redirects on the symbols {, , and . The only restriction is that these symbols then should not be used in filenames. With these changes, the user can choose to let either DOS <, >, >> or C {, . do the redirecting. The modified version of MAIN.C is compiled to obtain a new MAIN.OBJ, which can either be put into the library MC.LIB to replace the original MAIN by using the LIB.EXE utility, i.e. LIB MC.LIB MAINMAIN or it can be kept separate. If kept separate, remember to include it in the list of .OBJ files specified in the LINK call, i.e. LINK cmain myprogram. The three lines to change in Microsoft C's MAIN are: case '{': case ' ': if (*line == ' ') PC/MS-DOS Tips (2.0/2.1) 10 The following is a summary of some undocumented DOS 2.0 functions which can be invoked through interrupt 21H. The information provided herein have inaccuracies, so use it at our own risk! It is correct to the best of my knowledge. See the section in your DOS 2.0 manual entitled "Invoking DOS Functions" for further information. The function number provided below for each operation is to be placed in the AH register as described in the DOS manual. All numbers shown are in hex. FUNCTION DESCRIPTION 37 This interrupt is used to change incompatible configuration parameters to allow for switch indicators and whether hardware devices are available at every level of the tree directory. Usage: MOV AH, 37 MOV AL, func ; function code MOV DL, data INT 21H ; read function data is returned in DL Function code for AL: 0 - Return the DOS switch character in DL. Many systems might return "-". 1 - Make the character in DL the switch character. 2 - Read the device availability byte into DL. A 0 means devices that devices must be accessed in file I/O calls by /DEV/device. A non-zero value means that devices are accessible at every level of the directory tree (e.g., PRN is the printer and not a file PRN). 3 - Set the device availability byte to the contents of DL. Possible errors returned in AL: FF - Illegal function code specified in AL. 1F (?) Retrieve the pointer to the default drive parameter block. Usage: MOV AH, 1F INT 21H ; address of drive parameter block is returned in DS:BX PC/MS-DOS Tips (2.0/2.1) 11 32 (?) Retrieve the pointer to the drive parameter block for the drive number in DL, where 0 = default drive, 1 = drive A:, 2 = drive B:, etc.). Usage: MOV AH, 32h MOV DL, drivenum INT 21H ; address of drive parameter block is returned in DS:BX ; AL contains FF if the drive # in DL is invalid. F8 (?) Set OEM handler for INT 21H calls from F9 through FF to DS:DX. To reset these calls, pass DS and DX with FFFF. DOS is set up to allow ONE handler for all 7 of these calls. Any call to these handlers will result in the carry bit being set and AX will contain 1 if they are not initialized. The handling routine is passed all registers just as the user set them. The OEM handler routine should be exited through an IRET. Usage: LDS DX, handleaddr MOV AH, F8h INT 21H 4B Load and possibly execute a program (EXEC). This call is PARTIALLY documented in the IBM DOS 2.0 manual, but several function call values (for register AL) are omitted. They are: 1 - Create the program segment prefix and load the program, but do not begin execution. The CS:IP and SS:SP of the program are placed in the area provided by the user. +-------------------------------+ | Word segment addr of environ. | +-------------------------------+ | Dword ptr to cmd line at 80h | +-------------------------------+ | Dword ptr to default FCB to | | be passed at 5Ch. | +-------------------------------+ | Dword ptr to default FCD to | | be passed at 6Ch. | +-------------------------------+ | Dword value of SS:SP returned | +-------------------------------+ | Dword value of CS:IP returned | +-------------------------------+ 2 - This function still remains a mystery. PC/MS-DOS Tips (2.0/2.1) 12 4E Find first matching file (FIND FIRST). This function is PARTIALLY documented in the IBM DOS 2.0 manual. The description of what is returned in the DTA where the first 21 bytes are "reserved for DOS use on subsequent find next calls" contain the following in this order: 1 byte - attribute byte of search 1 byte - drive used in search 11 bytes- The search name used 2 bytes - Word value of last entry 4 bytes - Dword pointer to this DTA 2 bytes - Word directory start ----------------------------------- { The documented bytes follow here such as attribute found, file's time, date, size, and name found. 50 Define the current DTA (?) - this is all I know... 51 Retrieve current DTA (?) 52 Retrieve "IVARS" (?) 53 Define something about a DTA (?) 55 Duplicate a DTA (?) The DOS critical section flag may be interrogated from within an interrupt handler before requesting DOS services: MOV AH,34h INT 21h returns in ES:BX the address of a byte indicating (when set) that DOS is in an uninterruptible state, and no DOS calls should be made. PC/MS-DOS Tips (2.0/2.1) 13 To access DOS' PRINT capabilities: MOV AH,func INT 2Fh Where: AH = 0 adds the file specified by DS:DX to the print queue. DS:DX must point to valid opened FCB. AH = 1 cancels the file indicated by DS:DX. DS:DX must point to an FCB, opened or unopened. The drive byte must not be 0. Wildcards are restricted: ? is okay, * isn't. AH > 1 do nothing. Return with registers set as follows: DS,SI,DI,CX preserved, all others destroyed. AH = number of files currently in queue. AL = for AH=0, return 1 if queue was full. For all other cases, return 0. ES:BX = pointer to list of 10 FCBs in queue, 38 bytes/FCB. If the first byte of an FCB is -1, that FCB is unused. ES:DX = pointer to currently printing FCB. If the queue is empty, DX = -1. PC/MS-DOS Tips (2.0/2.1) 14 An Operating System Dialog - PC/MS-DOS This is a list of PCDOS and MSDOS peculiarities, etc. You will have to guess from the text what the particular questions were, but you systems programmers will find it worth your while to rummage through it all. There is a problem of compatibility between MS-DOS and IBM PC-DOS having to do with FCB Open and Create which has finally been tracked. The IBM 1.0, 1.1, and 2.0 documentation of OPEN (call 0FH) contains the following statement. "The current block field (FCB bytes C-D) is set to zero [when an FB is opened]." This statement is NOT true of MS-DOS 1.25 or MS-DOS 2.00. The difference is intentional, and the reason is CP/M 1.4 compatibility. Zeroing that field is not CP/M compatible. Some CP/M programs will not run when machine translated if that field is zero ed. The reason it is zeroed in the IBM versions is that IBM specifically requested that it be zeroed. This is the reason for the complaints from some vendors about the fact that IBM MultiPlan will not run under MS-DOS. It is probably the reason that some other IBM programs don't run under MS-DOS. PROGRAMMERS NOTE: Do what all MS/PC-DOS Systems programs do: Set every single FCB field you want to use regardless of what the documentation says is initialized. a) It seems there is a maximum of 19 handles, no matter what the files parameter is set to. Is this really the case? What does one gain by setting files to less than 19? Is memory for handles allocated dynamically? There is a maximum of 20 handles per process no matter what the files parameter is set to. There has to be a table in your process header for your handles, there is a limited amount of space down there. 40 bytes or so are taken up by each system FCB, setting files to less than 20 saves a little bit of memory. Memory for handles is not allocated dynamically, in general an attempt to do so would fail anyway. Recall that .COM files and most .EXE files are given the biggest piece of memory possible w hen they are EXECed because the DOS cannot make assumptions about how much memory these programs really use. This means they typically get ALL of the free memory, that means there is no free memory to allocate dynamically. You will find that almost a ll operating systems (CP/M is about the only exception) have a limit on the number of open files. MS-DOS has a limit of 20. CP/M has no such limits because it requires the user to keep FCBs in his own address space, managing FCBs is a pain. You get si mplicity and you give up very little. What program needs more than 20 open files? If you can think of one, it is probably a poorly written program in that it probably only needs a few open files at a time and doesn't bother to close files after it's d one with them. PC/MS-DOS Tips (2.0/2.1) 15 b) Execing a program eats 5 (I think) handles per try. Is this the passing of parent's environment that is mentioned (very briefly) in the documentatio n? What are these handles? They don't seem to be allocated with system calls, either. Is that true? std-in, std-out, std-err, std-aux, std-prn. I suppose you would like your program to be able to use the 1-12 system calls? That means there have to be 5, the first three are standard UNIX style fair and are required for the software tools approach to programming. Std-aux and Std-prn are required for system calls 3,4, and 5. Just because you are handed these default handles doesn't mean you can't close them. "... 1) what exactly is meant by the dos being in an unstable state. (This is what the documentation says happens if one returns to a user program directly from an int 24 handler.)..." It means that the DOS has the notion of an error being in effect. All printer echoing is turned off, and some other stuff doesn't work. Also, there are dirty buffers that are not correctly flushed out. Thus the disks may not be consistent. "... 2) my experimentation shows that an abort from a hard error handler causes an int 22 without the value for int 22 being sucked out of the program header. every other way out of a program uses he terminate address in the header. is this difference intentional? Why? DOes one expect the value at int 22 to be different from the value in the header ever?..." False. No INT 22h is ever issued. The header is only used to save the previous process' vectors. The address contained in INT 22 is saved in a temp spot, the contents of INTs 22-24 are restored from the header, and then an indirect jump is taken th rough the temp location. Certain programs (such as COMMAND) may want to intercept themselves from terminawill then attempt to write it out, causing a write-protect error. If you catch the INT 24 and do not return, the dirty b uffer still exists. To clear out the dirty buffer, you MUST return from the INT 24 saying to abort the process. You can then catch the terminate and restore your stack (you will be running on your parent's stack). "... 1) Why does PCDOS exec function 3 (overlay) demand that there be some free memory that it can allocate...." It doesn't. IBM specifically requested that the Exec code be overlayable in the MSDOS. As a result, it lives in the transient piece of COMMAND.COM and gets loaded when needed: thus the requidement for enough free space to laod the Exec loader (about 1.5K). Under other MSDOS's there is no such problem as the Exec system call lives in system space. A general rule of thumb is: if you are not going to use some space, free it. You can do this either via SetBlock system call, or by twiddling the EXE file he ader. You should avoid .COM format files. PC/MS-DOS Tips (2.0/2.1) 16 "... what happens if I try to overlay an .EXE file with the high/low switch set to load the thing for high memory..." Nothing. The HIGH/LOW switch is only for process creation, not for overlays. "... Are all these answeres the same for MSDOS?..." [Yes.] "... zeroing of the current record field ..." That incompatability existed between 1.1 PC-DOS and 1.25 MSDOS. 2.0 versions of both function identically (like 1.1 PC-DOS). The Shell command on PC-DOS 2.0 works just fine. CONFIG.SYS shell = b:\command.com b:\ /P Putting a disk with command.com in drive B: when the system boots causes COMMAND to be read from drive B: and the COMSPEC in the environment is "B:\COMMAND.COM". If you are having trouble it's because you are doing something wrong. Recall that your g iven COMSPEC is checked, if you give it a bad one it will try to go back to the default which is the root directory on the default drive. Recall also that this is an undocumented 2.0 feature so even if it doesn't work nobody is going to be all that ho t to do something about it. My error on the shell stuff, IBM hid it real well. The "b:\" is also documented on page 10-9 (the [d:][path] part). They did an equally poor job here. The /P and the path spec have absolutely nothing to do with the SHELL command, they are arguments to command. Expecting SHELL to know stuff particular to command is not reasonable because you are not restricted to running command as your top level she ll. You can run DEBUG as your top level shell by saying SHELL = debug.com But watch out!! debug is not designed to run as a top level shell. if you ever say "q" to this debug the system will crash. Command on the other hand is smart enough to run as a top level shell. If you give the /P switch to it Command does some specia l things to insure that typing EXIT to it will not cause the system to crash as with debug. There is absolutely no way for command to assume the /P switch because he must run as a top level shell, and as a utility. The smart user has to tell him what to do. Similarly the "b:\" tells command where to look for himself. For instance: SHELL = A:\BIN\COMMAND.COM D:\COMMAND\BIN /P The "A:\BIN\COMMAND.COM" tells SYSINIT where to load the initial command.com, the "D:\COMMAND\BIN" tells command where to look for himself when he needs to locate his transient. As you can see they are not restricted to being the same things. I suggest you foreward any complaints about the manual to [no address given] PC/MS-DOS Tips (2.0/2.1) 17 The volume ID attribute is very special, and is treated differently from all the other attributes. It is very "sticky", in order to find one you must look for it and it alone. And when you do look for it, you find only it and nothing else. The volume ID is constrained to be in the ROOT directory, and there can be only one file in the ROOT with the attribute. The FCB flavor calls have special code to enforce these rules. The new calls were supposed to enforce the same rules, but they are not working correctly, and unexpected results are possible. You should use the old FCB calls to diddle with volume ID for the moment. Thanks for the info. The causes of your problems are: (a) ^Z on output to a device in cooked mode will terminate the output. This is for CPM-compatability: you don't want stuff after the ^Z output to your printer for example. As a result COMMAND.COM issues a write to stdout and then checks to see if the number written is equal to the number requested. If they are not the same, then a redirection error is assumed. ECHO ^Z is supposed to output a single cha racter. It outputs NO characters and thus the strange message. Programs that use old function calls, get redirected, and then read more than is expected will behave bizarrely: how do you indicate EOF on a read-byte-from- console system call? I believe that it returns ^Z. Most of these programs we re never expecting to get redirected and thus, the failure to handle the boundary conditions properly. (b) The main crock about CP/M is that the extention on a file name determined the type of the file. This is bogus: a file should be distinguished by its contents, not by its name. When you are loading a file with the name *.EXE, it d oes NOT assume that it is an EXE format file. It looks at the first two bytes for a signature telling it that it is an EXE file. If it has the proper signature, then the loa proceeds. otherwise, it presumes the file to be a COM- form at file. If the file has the EXE signature, then the internal consistency IS checked. Pre-2.0 versions of MSDOS did not check the signature byte for EXE files. PC/MS-DOS Tips (2.0/2.1) 18 ;This routine implements the UN*X system call (in spite of the fact that I ;called it exec, the name in the DOS manual) under Lattice C although it ;should be easy to adopt it to other languages. One caution, it uses the ;undocumented 037h DOS call to retrieve the switch character. This is ; marked in the code and I will change it after DOS 2.0 so I'd rather ;the DOS purists don't jump down my throat about this. If you don't like it ;the way it is, change it. ; ; ; Darrell Plank ; BTL-IH PAGE 55,132 ; ; This function is modified from the macro by Brad Davis (b-davis@utah-cs) ; The prolog and epilog are modified from Jim Holtman's macros for Pascal. ; ; exec( cmd) ; char *md; ; ; This function accepts a string with the pathname of a command to be ; executed and executes it. The returned value is one of the following: ; 0: Successful ; -1: Insufficient Memory ; -2: Access Denied ; -3: No such command ; -4: Invalid command format ; PROLOG MACRO PUSH BP MOV BP,SP ENDM EPILOG MACRO NUM POP BP RET ENDM PC/MS-DOS Tips (2.0/2.1) 19 EXECVAL EQU 0 OVLVAL EQU 3 FNCINT EQU 21H SETBLK EQU 4AH EXECF EQU 4BH CR EQU 0DH PSP STRUC INTVECT DW ? TOM DW ? RES1 DB ? DOSLONG DB 5 DUP (?) TERMINA DD ? CTRLBRK DD ? CRITERR DD ? DOS1 DB 22 DUP (?) ENVIRO DW ? DOS2 DB 46 DUP (?) FPA1 DB 16 DUP (?) FPA2 DB 20 DUP (?) UPA DB 128 DUP (?) PSP ENDS EXECDEF STRUC NENVIRO DW COMMND DW 2 DUP (0) FCB5CH DW 2 DUP (0) FCB6CH DW 2 DUP (0) EXECDEF ENDS PC/MS-DOS Tips (2.0/2.1) 20 PGROUP GROUP PROG PROG SEGMENT BYTE PUBLIC 'prog' ASSUME CS:PGROUP PUBLIC EXEC EXEC PROC NEAR PROLOG PUSH DS PUSH ES ; ; free up as much memory as we can ; MOV AX,CS PUSH ES ; Save ES for later MOV ES,AX MOV BX,SS SUB BX,AX ADD BX,1000H ; 64K for stack segment MOV AH,SETBLK INT FNCINT JNC NEAR PTR LBL1 MOV AX,-1 ; Insufficient memory JMP NEAR PTR FINE LBL1: POP ES ; Get ES's original value ; ; Save SS and SP registers ; MOV CS:SPSAVE,SP MOV CS:SSSAVE,SS ; ; set up the parameter block ; MOV CS:EXECBLK.NENVIRO,0 ; Inherit envir. from parent MOV AX,3700H ; Undocumented call for SWITCHAR PC/MS-DOS Tips (2.0/2.1) 21 ; W A R N I N G: The following function call is undocumented and is ; liable to disappear or change in future versions of DOS. INT FNCINT MOV CS:COMMAND[1],DL ; Switchar MOV DX,4[BP] ; Address of the command MOV SI,DX MOV DI,DX CLD XOR AL,AL MOV CX,100H ; Longest string can be 100h REPNE SCASB ; Find Null termination SUB DX,DI NEG DX MOV CX,DX DEC CX ADD DX,2 MOV CS:COMMAND[0],DL ; Save command length LEA DI,CS:COMMAND[4] MOV AX,CS MOV ES,AX REP MOVSB ; Copy command into our buffer ASSUME DS:PGROUP MOV AX,CS MOV DS,AX ; DS points at code segment MOV BYTE PTR [DI],CR ; Put in Carriage Return LEA DX,COMMAND MOV EXECBLK.COMMND[0],DX MOV EXECBLK.COMMND[2],DS MOV BX,OFFSET EXECBLK XOR SI,SI MOV DS,[SI].ENVIRO ; Get environment address ASSUME DS:NOTHING LEA SI,CS:COMSPEC ; Point SI at env. variable name PUSH DS ; Swap PUSH ES ; ES POP DS ; and POP ES ; DS CALL GETENV PUSH DS ; Swap PUSH ES ; them POP DS ; back POP ES ; again MOV AH,EXECF MOV AL,EXECVAL ; OVLVAL here for overlay INT FNCINT JC NEAR PTR LBL2 MOV AX,0 ; Successful exec JMP NEAR PTR FINE LBL2: MOV SI,AX MOV AL,CS:ERRORS[SI] ; Get error code MOV AH,0FFH ; Sign extension - Assume Negative FINE: MOV SS,CS:SSSAVE MOV SP,CS:SPSAVE POP ES POP DS EPILOG 1 PC/MS-DOS Tips (2.0/2.1) 22 EXECBLK EXECDEF <> ; ; first byte of command s length excluding the length byte and the ; trailing \r. Second byte is switchar. ; COMMAND DB 2 DUP(?),"C ",254 DUP(?) COMSPEC DB "COMSPEC",0 SPSAVE DW SSSAVE DW ERRORS DB ? DB ? DB -3 ; No such command DB ? DB ? DB -2 ; Access denied DB ? DB ? DB -1 ; Insufficient memory DB ? DB ? DB -4 ; Invalid command format EXEC ENDP ; ; Getenv expects ES to have the environment paragraph and DS:SI to point ; to an ASCIIZ string with the desired environment variable in it. ; It returns the address of the proper string in ds:dx. ; PUBLIC GETENV GETENV PROC NEAR PROLOG PUSH AX PUSH CX PUSH SI PUSH DI MOV CS:VARNAME,SI ; Save offset of env. name XOR DI,DI PC/MS-DOS Tips (2.0/2.1) 23 ; ; At this point ds:si points to dummy variable environment name and ; es:di points to environment. ; CLD ;Forward string operations TOP: LODSB ;Get a char. of env. name CMP AL,0 ;If we're at the end JNE NEAR PTR LBL3 CMP BYTE PTR ES:[DI],'=' ;Check for match JNE NEAR PTR LBL4 ; ; We matched ; INC DI ;Move beyond '=' MOV DX,DI POP DI POP SI POP CX POP AX EPILOG 2 LBL4: ; ; At this point we found the end of the Env. variable name but it didn't ; match because the env. string was too long ; MOV CX,-1 REPNE SCASB ;Find the end of the env. string CMP BYTE PTR ES:[DI],0 JNE LBL3 MOV AX,-1 ;End of evironment area POP DI POP SI POP CX POP AX EPILOG 2 PC/MS-DOS Tips (2.0/2.1) 24 LBL3: ; ; Check if the next character matches ; AND AX,11011111b ;Capitalize the character in ax SCASB JE TOP ; ; If we get here we don't have a match so move on ; MOV SI,CS:VARNAME ;Go back to start of env. string XOR AX,AX MOV CX,-1 REPNE SCASB ;Go to next env. variable CMP BYTE PTR ES:[DI],0 JNE TOP MOV AX,-1 ;End of environment area POP DI POP SI POP CX POP AX EPILOG 2 VARNAME DW ? GETENV ENDP PROG ENDS END [This program will be kept in <INFO-IBMPC> as EXEC.ASM -ed] PC/MS-DOS Tips (2.0/2.1) 25 +----------------------------------------------------------------------------+ | D O S 2 . 1 I N T E R R U P T 2 1 F U N C T I O N C O D E S | +----------------------------------------------------------------------------+ +----+--------------------------+-----------------------+--------------------+ | AH | F U N C T I O N | Entry / Register Use | N O T E S | +----+--------------------------+-----------------------+--------------------+ | 00 | Program terminate | CS=PSP seg. addr. | Exit vectors are | | restored | | 01 | Keyboard input | N/A: returns AL | waits for a char; | | echoes it | | 02 | Display output | puts DL | break checked for | | 03 | AUX (Asynch) input | N/A: returns AL | Unbuffered, | non-interrupt | | 04 | AUX (Asynch) output | puts DL | " " | | 05 | Printer output | puts DL | | | 06 | Direct CON: I/O | DL=FF input | ZF set for no input| | 07 | Direct CON: input Noecho | N/A: returns AL | | | 08 | Console input Noecho | N/A: returns AL | Same as Fctn 1 | | except no echo | | 09 | Print string | DS:DX ==>string | string terminator $| | 0A | Buffered keyboard input | DS:DX ==>buffer | 1st byte is length | | 0B | Check std. input status | AL=FF if input | AL=00 if no input | | 0C | Clear kybd buf. + other | AL = function no. | fctns 1,6,7,8,A | +----+--------------------------+-----------------------+--------------------+ | 0D | Disk reset | N/A | Flushes all buffers| | 0E | Select disk | DL=drive no. | AL=no drives | | | on system | | 0F | Open file | DS:DX ==> FCB | AL=FF for error | | 10 | Close file | DS:DX ==> FCB | AL=FF for error | | 11 | Search for first entry | DS:DX ==> FCB | AL=FF for no match | | 12 | Search for next entry | DS:DX ==> FCB | must follow fctn 11| | 13 | Delete file | DS:DX ==> FCB | AL=FF for error | | 14 | Sequential read | DS:DX ==> FCB | EOF = 01 or 03 | | 15 | Sequential write | DS:DX ==> FCB | AL=01 (full) | | | 02 (bad buffer) | | 16 | Create file | DS:DX ==> FCB | AL=FF | | directory full | | 17 | Rename file | DS:DX ==> FCB | AL=FF for error | | 18 | Used internally by DOS: | | | | 19 | Current disk | N/A: returns AL | | | 1A | Set DTA | DS:DX ==> new DTA | | | 1B | Get FAT (default drive) | N/A: returns DS:BX | FAT id byte for | | | default drive | | 1C | Get FAT (select drive) | N/A: returns DS:BX | FAT id byte for DL | | 1D | Used internally by DOS: | | | | 1E | Used internally by DOS: | | | | 1F | Used internally by DOS: | | | | 20 | Used internally by DOS: | | | | 21 | Random read | DS:DX ==> FCB | AL=00 good read | | 22 | Random write | DS:DX ==> FCB | AL=00 good write | | 23 | File size | DS:DX ==> FCB | AL=00 rrf=# records| | 24 | Set random record field | DS:DX ==> FCB | | +----+--------------------------+-----------------------+--------------------+ | 25 | Set interrupt vector | DS:DX = vector | Int specified in AL| | 26 | Create new PSP | DX = segment no. | Use 4B instead | +----+--------------------------+-----------------------+--------------------+ | 27 | Random block read | DS:DX ==> FCB | CX = record count | | 28 | Random block write | DS:DX ==> FCB | CX = record count | | 29 | Parse filename | DS:SI ==> command | AL = parse service | +----+--------------------------+-----------------------+--------------------+ | 2A | Get date | N/A: returns CX:DX | CX=yr DH=mo DL=day | | 2B | Set date | CX:DX = new date | | | 2C | Get time | N/A: returns CX:DX | CH=hr CL=min DH=sec| | 2D | Set time | CX:DX = new time | | | 2E | Set / reset VERIFY | DL=0, AL=1/0 (on/off | See 54 | +----+--------------------------+-----------------------+--------------------+ | 2F | Get DTA | N/A: ES:BX ==>DTA | | | | 30 | Get DOS version number | N/A: returns AX | AL=major AH=minor | | 31 | Stay res. terminate | AL=retcode DX=size | | | 32 | Used internally by DOS: | | | | 33 | Ctrl-Break check | AL=00/01 (get/set) | | | | | DL=00/01 (off/on) | | | 34 | Used internally by DOS: | | | | 35 | Get interrupt vector | N/A: returns ES:BX | See 25 | | 36 | Get disk free space | DL=drive; returns BX | DX=tot CX=bytes | | | AX=sectors | | 37 | Used internally by DOS: | Get/set SWITCHAR | AL=0/1 DL=char | | 38 | Get natl dependent info | DS:DX ==> memory | country dependent | +----+--------------------------+-----------------------+--------------------+ | 39 | MKDIR - Create subdir. | DS:DX ==> ASCIIZ | Errors 3,5 | | 3A | RMDIR - Remove subdir. | DS:DX ==> ASCIIZ | Errors 3,5 | | 3B | CHDIR - Change subdir. | DS:DX ==> ASCIIZ | Error 3 | +----+--------------------------+-----------------------+--------------------+ | 3C | Create a file (handle) | DS:DX ==> ASCIIZ | CX=attr BX handle | | 3D | Open a file (handle) | DS:DX ==> ASCIIZ | AL=access code | | 3E | Close a file (handle) | BX = handle no. | Error 6 only | | 3F | Read (handle) | BX = handle no. | CX=read length | | 40 | Write (handle) | BX = handle no. | CX=write length | | 41 | Unlink - Dir. delete | DS:DX ==> ASCIIZ | Errors 2,5ength | | 42 | LSEEK - Move r/w pointer | BX = handle no. | AX = method | | | CX:DX = offset | | 43 | CHMOD - Change file mode | DS:DX ==> ASCIIZ | AL=function | | 44 | IOCTL - I/O control | BX = handle no. | AL=function value | | 45 | DUP - Dup file handle | BX = handle no. | AX new handle | | 46 | FDUP - Force dup handle | BX = handle no. | CX second handle | +----+--------------------------+-----------------------+--------------------+ | 47 | Get current directory | DL = drive | DS:SI==>64byte area| +----+--------------------------+-----------------------+--------------------+ | 48 | Allocate memory | BX = no. para. | AX block start | | 49 | Free memory | ES ==> block | Errors 7,9 | | 4A | Modify allocated memory | ES = block BX = size | Errors 7,8,9 | | 4B | EXEC - load a program | DS:DX ==>ASCIIZ | AL=function; 0 or 3| +----+--------------------------+-----------------------+--------------------+ | 4C | EXIT - Terminate | AL = retcode | See FC 0 | | 4D | WAIT - return code | N/A: returns AX | | | 4E | Find first match (ASCIIZ)| DS:DX ==> ASCIIZ | CX = attrib | | 4F | Find next match (ASCIIZ)| DTA ==> 4E call | same as 4E | | 50 | Used internally by DOS: | | | | 51 | Used internally by DOS: | | | | 52 | Used internally by DOS: | | | | 53 | Used internally by DOS: | | | | 54 | Get VERIFY state | N/A: AL=0/1 (on/off) | See FC 2E | | 55 | Used internally by DOS: | | | | 56 | Rename a file (ASCIIZ)| DS:DX ==> ASCIIZ | same drive only | | 57 | Get/Set file date stamp | BX = handle no. | DX=date CX=time | | | AL = 0/1 | +----+--------------------------+-----------------------+--------------------+ | AH | F U N C T I O N | Entry / Register Use | N O T E S | +----+--------------------------+-----------------------+--------------------+ [70205,1217] MASM.BUG 15-Apr-84 7975 10 Keywords: MASM ASSEMBLER PC-DOS BUGS This document describes some problems, omissionas, bugs and limitations in the IBM version of the Microsoft Macro Assembler. Disposition: <R D T): R The IBM / Microsoft Macro Assembler - Version 1.0 - Known Problems - and - Usage Hints - John Chapman [70205,1217] Introduction This document describes several known limitations, bugs and quirks of the IBM distributed Version (1.0) of the Microsoft Assembler. Note that these descriptions are from various sources, and have been verified at least once. Some of these problems are intermittent, dependent upon available memory, program size, structure of source program, etc. Disclaimer Please note that this document is provided without warrantee of any kind. Each individual must make the final determination of applicability to a specific situation, program, or configuration. Audience This document is intended for the experienced assembler language programmer. The reader is assumed to be familiar with the 8086 instruction set, the IBM Macro Assembler, and the IBM Personal Computer. Additions and Corrections This document will be periodically updated as new information is made available. If you have corrections, or additions, please send them to: John A. Chapman 844 S. Madison St. Hinsdale, Illinois 60521 Or: Send EMAIL to userid [70205,1217] on Compuserv, or MAIL to userid $AC on PCSHARE. Assembler Tips 2 Untrapped Errors When an instruction is accepted by the assembler, it is assumed that the instruction is not only syntactically correct, but that it will be decoded, as specified, into a legal machine operation. In MASM 1.0 this does not always occur: Segment Registers The 8088 does not support a compare with a segment register as the register operand. Coding CMP ES,0 will generate the instruction CMP AX,0 .... without generating a diagnostic message, or providing any other indication. Omitted Operands Most instructions with missing operands will generate error messages; the instruction: MOV AX, produces MOV AX,0 ... and no message Apparently, the above treatment occurs for all instructions that can use a register [non-segment register] with immediate data as the second operand. Zero is always used for the missing operand. Missing Brackets Erroneous code will also be generated if square brackets are omitted in certain operations, even thought an error message would be expected. MOV BYTE PTR ES:DI,'$' Is incorrect, and generates: MOV BYTE PTR ES:[7],'$' Rather than what was intended, which was: MOV BYTE PTR ES:[DI],'$' This appears to occur because the assembler finds DI in it's symbol table, equated to it's register triplet '111'b, and substitutes the 7 as if the programmer had said DI equ 7. Assembler Tips 3 Data entry errors Always scan comments in a newly entered program to assure that each comment is preceded by a semi-colon [;], rather than a colon [:]. The assembler will assume that the colon denotes a label, and will subsequently generate the spurious message "OPEN PROCEDURES", and may generate additional error messages. Review program labels in failing programs: the branch table sequence below provides both correct and incorrect examples. Correct Incorrect JMP CS:jumlist[BX] JMP CS:jumlist[BX] ... ... ... ... jumlist DW routin1 jumlist: DW routin1 DW routin2 DW routin2 Radix Specifications Even if a RADIX pseudo-op has been used to specify the default base for data values, the assembler will still check the last character for a valid radix specification, and use it if present. For example: SUB BX,0B generates: 83 EB 00 .. which is incorrect for X'0B"; while: SUB BX,11d or BX,0Bh .. are both correct; and will generate: 83 EB 0B .... which is the intended instruction. Pseudo-Operations The .XLIST pseudo operation will be ineffective [during both passes] if the command line parameter /D is specified for the assembly. The assembler will not correctly resolve 'identity' type definition errors in the EQUATE pseudo-op. LOOP-IT EQU LOOP-IT will cause the assembler to loop. Assembler Tips 4 Out of Memory Error in small programs Occasionally users will experience an MASM abort with the message "OUT OF MEMORY", with a large amount of installed, available memory, and a small program. This problem is a recognized bug common to Pascal, MASM, and other Microsoft products written in Pascal. A>ren masm.exe masm.xxx A>debug masm.xxx -s 0 ffff 81 fb 00 10 7e 03 xxxx:FB23 -d fb20,fb2f xxxx:FB20 00 -e FB27 xxxx FB27 7E.76 <===================Enter 76 in response to first part of line -w WRITING 10800 BYTES -q A> The patch above corrects an invalid comparison for memory size, caused by using JLE rather than JBE for the test. Assembler Tips 5 Reserved Words The IBM/Microsoft assembler documentation does not provide documentation on the reserved word list of the assembler. This is especially unfortunate, since the assembler will fail to provide a diagnostic message if a reserved word is used in place of an operand or label. Example: MOV ax,byte ;BYTE not defined - no error reported Pass 1 Errors omitted Errors caused in Pass 1 that are not repeated in pass 2 are not reported. Example: IF1 ABC EQU AX ENDIF -- Incorrect equate - no error. Macro Parameters If a macro is invoked within another macro, with a parameter which is a quoted string constant, all