An 8088 instruction

1
An 8088 instruction

• Label Instruction_mnemonic dest, src
  comment

The label is optional. It must always begin
with a letter and may contain only letters and
digits. It cannot duplicate a register name or
instruction mnemonic.
A three- or four-letter mnemonic indicating the
instruction to be performed
register, registeror register, memoryor memory,
registerNOT memory, memory
the assembler ignores everything after the
semicolon
2
Classes of 8088 instructions

• Load Accumulators or memory MOV, XCHG
• No flags are changed
• The source is unchanged
• Arithmetic
• ADD, ADC, SUB, SBB, MUL, DIV, INC, DEC, NEG,
  IMUL, IDIV
• Shift and Rotate
• ROL, ROR, RCL, RCR, SHL/SAL, SHR, SAR
• Logical
• AND, OR, XOR, NOT

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More Classes of 8088 instructions

• Controlling the execution flow
• JMP, CALL, RET, LOOP, Jxx
• I/O
• IN, OUT
• Changing the state of flags
• CLC, STC, CMC, CLI, STI
• Using data to set flags
• CMP, TEST
• Formatting data
• CBW, CWD
• Stack PUSH, POP

4
Fortran to Assembly

• Sum 0
• DO 19 I 1, 5
• Sum Sum I
• 19 CONTINUE

All Math occurs inside the 8088 For a variable,
either a memory location is dedicated, or a
register is dedicated
5
Fortran to Assembly

• Sum 0
• DO 19 I 1, 5
• Sum Sum I
• 19 CONTINUE

S the sign flag, the most significant bit of
the last result arithmetic, logical, shift and
rotate instructions CMP, TEST
Z the zero flag, ZF1 if the result was 0
arithmetic, logical, shift and rotate
instructions CMP, TEST
Note! Not all instructions change the flags.
For example, Mov does not change any flags.
6
Fortran to Assembly

• Sum 0
• DO 19 I 1, 5
• Sum Sum I
• 19 CONTINUE

C the carry flag, arithmetic, logical,
shift and rotate, CMP, ADC and SBB include
the carry flag in and - operations
O the overflow flag, OF1 if the last
arithmetic instruction resulted in an overflow
P the parity flag, PF1 if the last instruction
resulted in even parity (shown as e in D86)
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Carry vs. Overflow

• Be careful about the difference between carry and
  overflow with signed arithmetic
• 00000011 3 11111101 -3
• 00000000 0 CF 1 OF 0 SF 0 ZF1
• 01111110 126
• 00000011 3
• 10000001 -127 CF0 OF1 SF1 ZF0

8
Fortran to Assembly

• Sum 0
• DO 19 I 1, 5
• Sum Sum I
• 19 CONTINUE

a basic looping structure mov cx, 5 number
of times through the loop L1 denotes the top of
the loop instructions inside the loop go
here loop L1 terminates loop (like 19
continue)
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Fortran to Assembly

• Sum 0
• DO 19 I 1, 5
• Sum Sum I
• 19 CONTINUE

mov cx, 5 number of times through the
loop L1 loop L1 terminates loop (like 19
continue)
mov ax, 0 ax takes the role of sum
add ax,cx sum sum i
What are the contents of ax and cx at this
point? ax 5 4 3 2 1 15 cx 0
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a86 assembler

• gt a86 myfile.ext
• output is myfile.com, the machine language
  executable
• also myfile.sym the symbol names (like L1) for
  the debugger
• if there are errors
• error messages are written to myfile.ext
• error messages are marked with a tilda ()
• the error messages are removed by a86
• the original source (unmarked) is saved in
  myfile.old
• the assembler can catch syntax errors, but not
  logic errors

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d86 debugger

• gt d86 myfile (or gt d86 myfile.com)
• NOT d86 myfile.asm
• this command would be attempting to debug the
  ASCII text file, not the executable
• F1 is single step
• can examine each register and the flags after
  each step
• Any assembly language command can be executed in
  the debugger by simply typing the command
• Q to exit debugger

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Homework for Th. Aug. 26

• A) Write assembly language code to duplicate the
  function of the following Fortran
• N 7
• Sum 100
• DO 19 J 1 , N
• Sum Sum - JJ
• 19 Continue
• B) Assemble this program and examine execution
  of this program in the debugger. After exiting
  the loop, what arecx, SF, ZF, CF, OF, PF, and
  the register used for sum, ?