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Text File | 1985-11-09 | 13KB | 397 lines

MAKING PC SOFTWARE COMPATIBLE WITH PC jr WHILE MOST SOFTWARE WRITTEN FOR THE PC WILL RUN WITHOUT MODIFICATION ON PC JR SOME WILL CAUSE PROBLEMS BECAUSE OF THE DIFFERENCES BETWEEN THE MACHINES. THE MOST IMPORTANT DIFFERENCE BETWEEN THE PC AND PC JR IS THE VIDEO HARDWARE. THE MACHINES ARE ESENTIALLY EQUIVALENT WHEN PROGRAMMING DOWN TO THE BIOS CALL LEVEL, IF THE PROGRAMMER ATTEMPTS TO DIRECTLY MANIPULATE THE VIDEO HARDWARE ADDRESSES PROBLEMS WILL OCCUR. FOR EXAMPLE ; MODE SWITCHES ARE VERY EASILY ACCOMPLISHED ON THE PC, YOU MERELY WRITE THE MODE VALUE YOU WANT TO THE PORT AT ADDRESS 3D8 (THE MODE CONTROL REGISTER). WRITING A 00H TO THIS ADDRESS WILL PUT THE PC INTO 40 COLUMN X 25 LINES OF COLOR VIDEO MODE. WRITING A 0AH SETS UP THE 320 X 200 COLOR GRAPHICS MODE. WITH JUNIOR HOWEVER, IT IS NOT THAT SIMPLE. THE PORT AT ADDRESS 3D8 DOES NOT EXIST AND JUNIORS COMPLICATED VIDEO GATE ARRAY MAKES CHANGING MODES MUCH MORE DIFFICULT. IF A PROGRAM THAT RUNS FINE ON THE PC FAILS ON THE JUNIOR IT MAY BE BECAUSE OF THIS TYPE OF MODE CHANGING ERROR. TO FIND OUT IF THIS IS THE CASE YOU CAN LOOK AT THE CODE USING DEBUG. I MERELY SEARCH FOR THE MOST OBVIOUS WAYS THAT A MODE CHANGE CAN BE ACCOMPLISHED ON THE PC. THE FOLLOWING CODE IS AN EXAMPLE; MOV AL,XX ;WHERE XX IS THE NEW MODE VALUE TO BE SET MOV DX,3D8 ;SETUP ADDRESS OF THE MODE CONTROL PORT OUT DX,AL ;OUTPUT THE NEW VALUE SOMETIMES YOU WIL FIND THESE INSTRUCTIONS IN SLIGHTLY DIFERENT ODERINGS SUCH AS; MOV DX,3D8 MOV AL,XX OUT DX,AL BOTH OF THESE WILL DO THE SAME THING TO THE VIDEO HARDWARE. THEY WILL CHANGE THE MODE TO THE VALUE OF XX, WHATEVER THAT MAY BE. IN HEX THESE INSTRUCTIONS LOOK LIKE THIS; B0 XX ;THE MOVE AL,XX INSTRUCTION BA D8 03 ; MOV DX,3D8 EE ; OUT DX,AL THE EASIEST THING TO LOOK FOR IS THE MOV DX,3D8 INSTRUCTION. THE DEBUG COMMAND FOR THIS IS; -S STARTING ADDRESS ENDING ADDRESS SEARCH STRING FOR EXAMPLE; -S 0 FFFE BA D8 03 THIS WILL SEARCH FROM ADDRESS 0 TO ADDRESS FFFE IN THE CURRENT SEGMENT FOR THE MOV DX,3D8 INSTRUCTION. IF IT FINDS ONE DEBUG WILL DISPLAY THE ADDRESS WHERE IT CAN BE FOUND. LIKE SO; 901:186 901:1B7 THIS WOULD MEAN THAT DEBUG HAS FOUND TWO INSTANCES OF THE MOV DX,3D8 INSTRUCTION. LET'S LOOK AT ONE OF THEM. WE WILL USE THE DEBUG UNASSEMBLE COMMAND AND THE ADDRESS THAT DEBUG HAS PROVIDED. -U 901:187 THE RESULT OF THIS SHOULD LOOK SOMETHING LIKE THIS; 901:0187 BAD803 MOV DX,03D8 901:0189 B000 MOV AL,00 901:018B EE OUT DX,AL . . . . AND SO ON . THIS IS DEFINATELY ONE OF THE OFFENDING PROGRAMS. THE PROGRAMMER IS SETTING THE PC TO THE 40 X 25 COLOR ALPHANUMERIC DISPLAY. ON JUNIOR HOWEVER, THEY ARE NOT ACCOMPLISHING ANYTHING AT ALL. TO FIX THIS WE MUST MAKE SOME SIMPLE CHANGES TO THE CODE. INSTEAD OF DOING THE MODE CHANGE DIRECTLY WE WILL LET BIOS DO IT FOR US. ALL WE NEED TO DO IS TELL BIOS WHAT MODE WE WANT AND THEN DO A BIOS VIDEO CALL. THAT NEW CODE WILL LOOK LIKE THIS; MOV AX,0000H ;TELL BIOS WE WANT 40 X 25 MODE INT 10H ;CALL THE BIOS VIDEO ROUTINE WE PLACE THE VALUE 00 INTO THE AH REGISTER (THE AX 16 BIT REGISTER IS THE COMBINATION OF THE AH AND AL 8 BIT REGISTERS) . THIS TELLS BIOS THAT WE WANT IT TO DO A MODE CHANGE. THE VALUE WE PLACE IN THE AL REGISTER WILL TELL BIOS THE NEW MODE WE WANT. TO REPLACE THE OLD CODE WITH THE NEW WE WILL USE THE DEBUG ASSEMBLE COMMAND WHICH HAS THE FOLLOWING FORM; -A STARTING ADDRESS TO PUT THE NEW CODE WHERE THE OLD CODE WAS JUST USE THE SAME ADDRESS. -A 901:0186 DEBUG WILL RESPOND LIKE SO; 901:0186 _ FROM HERE WE JUST TYPE IN THE INSTRUCTIONS. 901:0186 MOV AX,0000 901:0189 INT 10H 901:018B _ IN ORDER TO WIPE OUT THE out dx,al INSTRUCTION, WHICH INSN'T NNEDED ANYMORE, WE WILL ADD A nop (DO NOTHING) COMMAND TO WRITE OVER IT. 901:018B NOP NOW IF WE WERE TO ONCE AGAIN DO AN UNASSEMBLE COMMAND WE COULD SEE ALL THE CHANGES WE HAVE MADE. -U 901:0186 901:0186 B80100 MOV AX,0000 901:0189 CD10 INT 10 901:018B 90 NOP . . . . THIS ALL SHOULD STAY THE SAME . . . NOT ONLY WILL THIS CODE NOW FUNCTION PROPERLY ON JUNIOR, IT WILL STILL OPERATE ON THE PC AS WELL. THIS IS BECAUSE THE TWO MACHINES ARE COMPATIBLE AT THE BIOS CALL LEVEL. IT IS INTERESTING TO NOTE THAT THE NEW CODE COULD BE ONE BYTE SHORTER THAN THE OLD. THIS IS BECAUSE THE NOP WOULD NOT HAVE BEEN NEEDED IF THE PROGRAM HAD BEEN WRITTEN THIS WAY IN THE FIRST PLACE. THE MOST DIFFICULT PART OF MAKING THESE TYPES OF CHANGES IS CONVERTING FROM THE PC MODE VALUES (DESIGNATED XX ABOVE) TO THE BIOS MODE VALUES. THE FOLLOWING TABLE SUMMARIZES THE NEEDED CHANGES. PC MODE REGISTER VALUES (HEX) BIOS CALL MODE VALUES (HEX) 2C 00 40 x 25 b/w 28 01 40 x 25 color 2D 02 80 x 25 b/w 29 03 80 x 25 color 0A or 2A 04 320 x 200 4 color 0E or 2E 05 320 x 200 b/w 1E or 3E 06 640 x 200 b/w THERE ARE OTHER VALUES AVAILABLE FOR BOTH THE PC MODE REGISTER AND THE BIOS CALLS, ESPECIALLY ON THE JUNIOR. USING BIOS JUNIOR CAN BE PLACED INTO ONE OF 3 MODES THAT THE PC DOES NOT EVEN HAVE AVAILABLE. GOING BACK TO OUR ORIGINAL SEARCH FOR THE mov dx,3D8 INSTRUCTION USING DEBUG WE FOUND TWO INSTANCES IN THIS PROGRAM. LET'S LOOK AT THE SECOND EXAMPLE NOW. USE THE DEBUG UNASSEMBLE COMMAND AGAIN. -U 901:1B7 THIS WILL SHOW US THE FOLLOWING CODE; 0901:01B7 BAD803 MOV DX,03D8 0901:01BA B02A MOV AL,2A 0901:01BC 80FB00 CMP BL,00 0901:01BF 7502 JNZ 01C3 0901:01C1 B02E MOV AL,2E 0901:01C3 EE OUT DX,AL 0901:01C4 BAD903 MOV DX,03D9 0901:01C7 B030 MOV AL,30 0901:01C9 EE OUT DX,AL . . . THIS IS A SLIGHTLY MORE COMPLICATED EXAMPLE. HERE THE PROGRAM LOADS THE MODE REGISTER ADDRESS INTO DX AND THEN SETS UP A MODE VALUE OF 2A IN AL. IT THEN TESTS THE BL REGISTER TO SEE IF IT IS ZERO. IF IT IS NOT, THEN THE PROGRAM JUMPS TO THE out dx,al INSTRUCTION WHICH SETS THE MODE TO 320X200 COLOR GRAPHICS. IF BL WAS NOT ZERO THEN A NEW MODE VALUE IS PLACED INTO AL. THIS TIME THE MODE SELECTED IS 80 X 25 B/W ALPHANUMERICS (2E). THE NEXT INSTRUCTION (mov dx,3D9) PUTS THE ADDRESS OF THE PC'S COLOR SELECT REGISTER INTO DX IN PREPARATION FOR THE out dx,al INSTRUCTION. BEFORE THAT HAPPENS THE AL REGISTER IS LOADED WITH THE COLOR SELECT VALUE (30). THE COLOR SELECT REGISTER AT ADDRESS 3D9 IS THE OTHER VIDEO REGISTER THAT JUNIOR DOES NOT HAVE. COLOR SELECTION CAN BE ACCOMPLISHED ON JUNIOR THROUGH A BIOS CALL JUST LIKE THE MODE SELECTS. TO MAKE THIS CODE COMPATIBLE WITH JUNIOR WE MUST MAKE THE FOLLOWING CODE CHANGES: MOV AX,0004 ;SET BIOS MODE SET COMMAND AND 320 X 200 COLOR CMP BL,00 ;TEST BL FOR ZERO JNZ int ;JUMP IF NOT ZERO TO THE int INSTRUCTION MOV AL,05 ;SET MODE TO 320 X 200 B/W GRAPHICS INT 10H ;BIOS VIDEO CALL THIS IS ALL WE NEED TO MAKE THE MODE CHANGES ON JUNIOR. BUT, THIS CODE REQUIRES 13 BYTES WHICH IS ONE BYTE SHORTER, FORTUNATELY, THAN THE EXISTING PC CODE. UNFORTUNATELY, THE COLOR SELECT CODE THAT FOLLOWS IT WILL USE UP ONE MORE BYTE. SO THE TWO CHANGES TOGETHER WILL JUST FIT INTO THE SAME SPACE. THE CODE CHANGE FOR THE COLOR SELECT CHANGE IS: MOV AH,0B ;SET THE BIOS COLOR SELECT COMMAND BYTE MOV BX,0001 ;SELECTS CYAN, MAGENTA, AND WHITE COLORS INT 10H ;BIOS VIDEO CALL USING DEBUG TO ASSEMBLE THE REPLACEMENT CODE; -A 901:1B7 0901:01B7 MOV AX,0004 0901:01BA CMP BL,00 0901:01BD JNC 01C1 0901:01BF MOV AL,05 0901:01C1 INT 10H 0901:01C3 MOV AH,0B 0901:01C5 MOV BX,0001 0901:01C8 INT 10H AFTER YOU MAKE ALL THE FIXES DO A WRITE COMMAND TO HAVE DEBUG SAVE THE CHANGES ON THE DISK. THESE TYPES OF PROBLEMS CAN USUALLY BE FIXED WITH THE SAME OR FEWER BYTES OF CODE. IF THE FIX REQUIRES MORE CODE THAN YOU HAVE SPACE FOR IT BECOMES A REAL HEADACHE TO TRY AND FIX. YOU MUST MAKE CORRECTIONS IN ALL THE CALL AND JUMP INSTRUCTIONS. ANY INSTRUCTION THAT USES AN OFFSET MIGHT ALSO HAVE TO BE REPAIRED. IN A LONG PROGRAM THIS CAN BECOME EXTREMELY DIFFICULT. BUT IN WELL WRITTEN CODE THESE TYPES OF MODE AND COLOR CHANGES HAPPEN ONLY RARELY. ANOTHER DIFFERENCE BETWEEN PC AND JUNIOR CAN BE FOUND IN THE BIOS ROMS. THE NEXT TO LAST BYTE (F000:FFFE) CONTAINS A SIGNATURE BYTE WHICH CAN TELL A PROGRAM IF IT IS RUNNING IN A JUNIOR, A PC, OR A PC XT. FOR JUNIOR THIS BYTE IS FDH, FOR PC IT IS FFH, AND THE XT HAS AN FEH AT THIS LOCATION. AT PRESENT THE ONLY PLACE I HAVE SEEN THIS USED IS IN DOS 2.1, COMMAND.COM. USE DEBUG TO LOAD COMMAND.COM AND SEARCH FOR A CMP BYTE PTR [FFFE],FD INSTRUCTION. A>DEBUG COMMAND.COM -S 0 FFFE 80 3E FE FF FD 0905:354B IF YOU UNASSEMBLE THIS COMMAND YOU WILL FIND THAT WHAT IS HAPPENING HERE IS THAT WHEN COMMAND.COM DETERMINES IT IS IN A JUNIOR IT WILL SCAN THE CARTRIDGE ADDRESS SPACE LOOKING FOR PROGRAMS THERE. SINCE THE PC DOESN'T HAVE CARTRIDGES IT NEED TO DO THIS IF IT IS RUNNING IN A PC. THIS EXPLAINS WHY WHEN YOU HAVE THE CARTRIDGE BASIC INSTALLED AND YOU TYPE THE basica COMMAND YOU GO INTO BASIC WITHOUT DOING A DISK ACCESS. NORMALLY COMMAND.COM WILL SEARCH THE DIRECTORY OF THE CURRENT DRIVE FOR ANY COMMAND THAT IS NOT INTERNAL. BUT WHEN IT IS RUNNING ON A JUNIOR IT WILL SCAN THE CARTRIDGE SPACE LOOKING FOR BASIC BEFORE IT GOES TO THE DISK. THIS MIGHT BE USED TO UPGRADE DOS WITH ALL SORTS OF NEW FUNCTIONS THAT ON JUNIOR WOULD SEEM TO BE INTERNAL TO COMMAND.COM. THESE NEW FUNCTIONS WOULD BE PROGRAMMED INTO A CARTRIDGE RATHER THAN MODIFYING DOS. THIS MIGHT MEAN THAT SOME DOS REVISIONS WOULD COME IN CARTRIDGE FORM IN THE FUTURE. NOT A BAD FEATURE WHEN YOU UNDERSTAND IT. OTHER STRANGE THINGS ABOUT JUNIOR'S VIDEO SECTION WILL COME LATER. AFTER YOU HAVE BECOME THOROUGHLY CONFUSED BY THE TECHNICAL REFERENCE MANUAL TUNE IN HERE AND WE WILL EXPLAIN "PAGES" AND "PLANES". HOW ABOUT "WILL THE REAL "PAGE" PLEASE STAND UP"?