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. The A86 Language and the A86 Program First, let's establish what we mean when we say A86. On one hand, A86 is the name for my assembly language for the Intel 86 family of (IBM-PC and compatible) computers. Statements written in this language are used to specify machine instructions for the 86 family and to allocate memory space for program data. On the other hand, A86 is the name for a program called an assembler, that translates these human readable statements into a machine readable form. The input to the assembler is a source file (or a list of source files) containing assembly language statements. The output of the assembler is a file containing binary program code that can either be run as a program on the PC, or combined with other modules (using a linker) to make a program. 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 4-2 The statements in an A86 source file are made up of keywords, identifiers, numbers, strings, special characters, and comments. A keyword is a symbol that has special meaning to the assembler, such as an instruction mnemonic (MOV, CALL) or some other reserved word in the assembly language (DB, SEGMENT, EQU). Identifiers are programmer-defined symbols, used to represent such things as variables, labels in the code, and numerical constants. Identifiers may contain letters, numbers, and the characters _, @, $, and ?, but must begin with a letter, _, or @. The identifier name is considered unique up to 127 characters, but it can be of any length (up to 255 characters). Examples of identifiers are: COUNT, L1, and A_BYTE. Numbers in A86 may be expressed as decimal, hexadecimal, octal, or binary. 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), and 10100110xB (binary). Strings are characters enclosed in single quotes. Examples of strings are: '1st string' and 'SIGN-ON MESSAGE, V1.0'. The single quote is one 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. For compatibility with other assemblers, I now also accept double quotes for strings. 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' 4-3 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. 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: 4-4 BYTE_VAR DB 0 ; A byte variable. WORD_VAR DW 0 ; A word variable. Labels 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 X: DW ?, the colon following the X means that X is a label; the instruction MOV SI,X moves the immediate constant address of X into the SI register. On the other hand, if you declare X DW ?, with no colon, then X is a word variable; the same instruction MOV SI,X now does something different: it loads the run-time value of the memory word X into the SI register. 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 an 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. 4-5 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.