PC Underground/UNDERGROUND.ISO/share/a86/a04.doc

Text File | 1994-01-19 | 11KB | 250 lines

CHAPTER 4   ELEMENTS OF THE A86 LANGUAGE

This chapter begins the description of the A86 language.  It's a
bit more tutorial in nature than the rest of the manual.  I'll
start by describing the elementary building blocks of the
language.


General Categories of A86 Elements

The statements in an A86 source file can be classified in three
general categories: instruction statements, data allocation
statements, and assembler directives.  An instruction statement
uses an easily remembered name (a mnemonic) and possibly one or
more operands to specify a machine instruction to be generated.
A data allocation statement reserves, and optionally initializes,
memory space for program data.  An assembler directive is a
statement that gives special instructions to the assembler.
Directives are unlike the instruction and data allocation
statements in that they do not specify the actual contents of
memory.  Examples of the three types of A86 statements are given
below.  These are provided to give you a general idea of what the
different kinds of statements look like.

Instruction Statements

MOV AX,BX
CALL SORT_PROCEDURE
ADD AL,7

Data Allocation Statements

A_VARIABLE DW 0
DB 'HELLO'

Assembler Directives

CODE SEGMENT
ITEM_COUNT EQU 5

The statements in an A86 source file are made up of reserved
symbols, user symbols, numbers, strings, special characters, and
comments.

Symbols are the "words" of the A86 language.  All symbols are a
collection of consecutive letters, numbers, and assorted special
characters: _, @, $, and ?.  Symbols cannot begin with digits:
anything that begins with a digit is a number.  Symbols can begin
with any of the special characters just listed.  Symbols can also
begin with a period, which is the only place within the symbol
name a period can appear.
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Reserved symbols have a built-in meaning to the assembler:
instruction mnemonics (MOV, CALL), directive names (DB, STRUC),
register names, expression operators, etc.  User symbols have
meanings defined by the programmer: program locations, variable
names, equated constants, etc.  The user symbol name is
considered unique up to 127 characters, but it can be of any
length (up to 255 characters).   Examples of user symbols are:
COUNT, L1, and A_BYTE.

Numbers in A86 may be expressed as decimal, hexadecimal, octal,
binary, or decimal "K".  These must begin with a decimal digit
and, except in the case of a decimal or hexadecimal number, must
end with "x" followed by a letter identifying the base of the
number.  A number without an identifying base is hexadecimal if
the first digit is 0; decimal if the first digit is 1 through 9.
Examples of A86 numbers are: 123 (decimal), 0ABC (hexadecimal),
1776xQ (octal), 10100110xB (binary), and 32K (decimal 32 times
1024).

Strings are characters enclosed in either single or double
quotes.  Examples of strings are: '1st string' and "SIGN-ON
MESSAGE, V1.0".  If you wish to include a quote mark within a
string, you can double it; for example, 'that''s nice' specifies
a single quote mark within the string.  The single quote and
double quote are two of many special characters used in the
assembly language.  Others, run together in a list, are: ! $ ? ;
: = , [ ] . + - ( ) * / >.  The space and tab characters are also
special characters, used as separators in the assembly language.

A comment is a sequence of characters used for program
documentation only; it is ignored by the assembler.  Comments
begin with a semicolon (;) and run to the end of the line on
which they are started.  Examples of lines with comments are
shown below:

; This entire line is a comment.
MOV AX,BX  ; This is a comment next to an instruction statement.

Alternatively, for compatibility with other assemblers, I provide
the COMMENT directive.  The next non-blank character after
COMMENT is a delimiter to a comment that can run across many
lines; all text is ignored, until a second instance of the
delimiter is seen.  For example,

COMMENT 'This comment
runs across two lines'

I don't like COMMENT, because I think it's very dangerous.  If,
for example, you have two COMMENTs in your program, and you
forget to close the first one, the assembler will happily ignore
all source code between the comments.  If that source code does
not happen to contain any labels referenced elsewhere, the error
may not be detected until your program blows up.  For multiline
comments, I urge you to simply start each line with a semicolon.
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Statements in the A86 are line oriented, which means that
statements may not be broken across line boundaries.  A86 source
lines may be entered in a free form fashion; that is, without
regard to the column orientation of the symbols and special
characters.

PLEASE NOTE: Because an A86 line is free formatted, there is no
need for you to put the operands to your instructions in a
separate column.  You organize things into columns when you want
to visually scan down the column; and you practically never scan
operands separate from their opcodes.  The only reason that 99%
of the assembly-language programs out there in the world have
operands in a separate column is that some IBM assembler written
back in 1953 required it.  It makes no sense to have operands in
a separate column, so STOP DOING IT!


Operand Typing and Code Generation

A86 is a strongly typed assembly language.   What this means is
that operands to instructions (registers, variables, labels,
constants) have a type attribute associated with them which tells
the assembler something about them.  For example, the operand 4
has type "number", which tells the assembler that it is a
numerical constant, rather than a register or an address in the
code or data.  The following discussion explains the types
associated with instruction operands and how this type
information is used to generate particular machine opcodes from
general purpose instruction mnemonics.

Registers

The 8086 has 8 general purpose word (two-byte) registers:
AX,BX,CX,DX,SI,DI,BP, and SP.  The first four of those registers
are subdivided into 8 general purpose one-byte registers
AH,AL,BH,BL,CH,CL,DH, and DL.  There are also 4 16-bit segment
registers CS,DS,ES, and SS, used for addressing memory; and the
implicit instruction-pointer register (referred to as IP,
although "IP" is not part of the A86 assembly language).

Variables

A variable is a unit of program data with a symbolic name,
residing at a specific location in 8086 memory.  A variable is
given a type at the time it is defined, which indicates the
number of bytes associated with its symbol.  Variables defined
with a DB statement are given type BYTE (one byte), and those
defined with the DW statement are given type WORD (two bytes).
Examples:

BYTE_VAR DB 0   ; A byte variable.
WORD_VAR DW 0   ; A word variable.

Labels
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A label is a symbol referring to a location in the program code.
It is defined as an identifier, followed by a colon (:), used to
represent the location of a particular instruction or data
structure.  Such a label may be on a line by itself or it may
immediately precede an instruction statement (on the same line).
In the following example, LABEL_1 and LABEL_2 are both labels for
the MOV AL,BL instruction.

LABEL_1:
LABEL_2: MOV AL,BL

In the A86 assembly language, labels have a type identical to
that of constants.  Thus, the instruction MOV BX,LABEL_2 is
accepted, and the code to move the immediate constant address of
LABEL2 into BX, is generated.

IMPORTANT: you must understand the distinction between a label
and a variable, because you may generate a different instruction
than you intended if you confuse them.  For example, if you
declare  XXX: DW ?, the colon following the XXX means that XXX is
a label; the instruction MOV SI,XXX moves the immediate constant
address of XXX into the SI register.  On the other hand, if you
declare XXX DW ?, with no colon, then XXX is a word variable; the
same instruction MOV SI,XXX now does something different: it
loads the run-time value of the memory word XXX into the SI
register.  You can override the definition of a symbol in any
usage with the immediate-value operator OFFSET or the
memory-variable opertors B,W,D,Q, or T.  Thus, MOV SI,OFFSET XXX
loads the immediate value pointing to XXX no matter how XXX was
declared; MOV SI,XXX W loads the word-variable at XXX no matter
how XXX was declared.

Constants

A constant is a numerical value computed from an assembly-time
expression.  For example, 123 and 3 + 2 - 1 both represent
constants.  A constant differs from a variable in that it
specifies a pure number, known by the assembler before the
program is run, rather than a number fetched from memory when the
program is running.


Generating Opcodes from General Purpose Mnemonics

My A86 assembly language is modeled after Intel's ASM86 language,
which uses general purpose mnemonics to represent classes of
machine instructions rather than having a different mnemonic for
each opcode.  For example, the MOV mnemonic is used for all of
the following: move byte register to byte register, load word
register from memory, load byte register with constant, move word
register to memory, move immediate value to word register, move
immediate value to memory, etc.  This feature saves you from
having to distinguish "move" from "load," "move constant" from
"move memory," "move byte" from "move word," etc.
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Because the same general purpose mnemonic can apply to several
different machine opcodes, A86 uses the type information
associated with an instruction's operands in determining the
particular opcode to produce.  The type information associated
with instruction operands is also used to discover programmer
errors, such as attempting to move a word register to a byte
register.

The examples that follow illustrate the use of operand types in
generating machine opcodes and discovering programmer errors.  In
each of the examples, the MOV instruction produces a different
8086 opcode, or an error.  The symbols used in the examples are
assumed to be defined as follows: BVAR is a byte variable, WVAR
is a word variable, and LAB is a label.  As you examine these MOV
instructions, notice that, in each case, the operand on the right
is considered to be the source and the operand on the left is the
destination.  This is a general rule that applies to all
two-operand instruction statements.

MOV AX,BX     ; (8B) Move word register to word register.
MOV AX,BL     ; ERROR: Type conflict (word,byte).
MOV CX,5      ; (B9) Move constant to word register.
MOV BVAR,AL   ; (A0) Move AL register to byte in memory.
MOV AL,WVAR   ; ERROR: Type conflict (byte,word).
MOV LAB,5     ; ERROR: Can't use label/constant as dest. to MOV.
MOV WVAR,SI   ; (89) Move word register to word in memory.
MOV BL,1024   ; ERROR: Constant is too large to fit in a byte.