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Physics 413 Chapter 8

Physics 413 Chapter 8. IBM PC Assemblers. An assembler is a program that takes your assembly language program and converts the instructions into op-codes that the microprocessor can understand.

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Physics 413 Chapter 8

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  1. Physics 413Chapter 8

  2. IBM PC Assemblers • An assembleris a program that takes your assembly language program and converts the instructions into op-codes that the microprocessor can understand. • The other job of the assembler is to calculate the number of steps that the processor may have to jump forward or backward if it encounters a transfer of control instruction like JNE or CALL. These two tasks are very tedious for a human and are best relegated to the assembler.

  3. DEBUG & MASM • The first one is a no-frills assembler called DEBUG. It is easier for the beginner to use and comes free with DOS for your IBM PC. • The other is expensive and comes with too many bells and whistles. It is called MASM • DOS utility exe2 bin converts .exe to .com • DEBUG cannot create .EXE (executable) files • DEBUG ischeaper and easier to learn to use for the beginner.

  4. DEBUG Type debug at the DOS prompt (C:>). All you will see is _ Q Exit from Debug R Displays the processor registers R AX Displays AX A Assemble. A 100 starts at CS:0100 D Dump (display or list) 128 bytes of memory E 100 Enter data. Space bar for next byte! F 100 120 AB Fill 0100 through 0120 with AB G = 100 Execute program starting 0100 P = 100 Single-step. Treats LOOP as a step T = 100 Single-step. Not good for subroutine U = 100 Unassemble displays mnemonics

  5. Assembler Directives • db is a directive that tells the assembler to store the byte-size numbers • db 2A 69 C4 5E will store these 4 bytes of data • db “ hello ” will store ASCII codes of h, e, l, l, o • equ assigns a value to a name • stuff equ 7

  6. What does MOV AL, TRICK do? • Depends!The answer depends on how TRICK was defined. • If your program had a statement TRICK EQU 6D then the number 6D will be copied into AL • If TRICK was defined as TRICK DB 2BH, 57H, 8EH then AL = 2B

  7. MASM Program .model small .stack .code mov cx, 7h mov si, offset mygoody mov al, 0 again:add al, [si] inc si dec cx jne again .exit .data mygoody db 57h, a3h, 26h, 4bh, 69h, e5h, 7ch end

  8. What does MOV DX, offset TRICK do? • offsetis another directive • If TRICK was defined as TRICK DB 2BH, 57H, 8EH then DX = logical address of 2B • If your program had a statement TRICK EQU 6D then error

  9. What does MOV DS, seg TRICK do? • segis another directive • If TRICK was defined as TRICK DB 2BH, 57H, 8EH then DS = segment of 2B • If your program had a statement TRICK EQU 6D then error

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