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Interpreting Data
Unless followed by a lower-case letter, all numeric output from the
inverse assembler is in hexadecimal format. Decimal values are
indicated by a lower-case "d" (as in the INT instruction).
Up to four instructions may be displayed for a single analyzer state
because the 80386DX can fetch a double word with four instruction
bytes from program memory. If the least significant byte of this double
word contains a single-byte instruction, the next sequential instruction
begins in the next higher byte. This process continues from the least
significant byte to the most significant byte until all of the bytes of the
fetched double word are used. When a single state contains more than
one instruction, each instruction will be displayed on a separate line.
For example:
+0015
+0016
=NEGECX
=
MOV BYTE
PI'R [EBP][ -lBH].Cl..
=
XXXXXXDAH READ MEM
OPCODE FETCH
MEMORY READ
Line number
+
0015 displays two instructions from a double-word.
Since instructions may begin in any byte position, the last bytes of a
multiple-byte instruction may extend into the lower bytes of the next
double word fetched. In this instance, the next sequential instruction
begins in the next higher byte of the next double-word after the
previous instruction and operands. When interpreting a given state,
the inverse assembler will ignore bytes used by a previous instruction
and will only display the instructions that begin in that state. For
example:
Byte
Position
3
2
1 0
01 20 BB 24 Single byte instruction MOV BX (BB) starts in byte 1.
10 BF D2 31 Double byte instruction XOR DX,DX (31D2) begins in byte 0
and continues into byte 1. Next instruction MOV DI (BF) begins
in byte2.
'F'7
C8 SC 00 Double byte instruction DIVDI (F7F7) starts in byte 3 and
continues into byte 0 of next double word fetched.
30
CA 81
'F'7
Next instruction ORDX (81CA) begins in byte 2 immediately
after last instruction.
Asterisks (*) in the inverse assembler output indicate that a portion (or
portions) of an instruction was not captured by the analyzer. Missing
opcodes occur frequently and are primarily due to microprocessor
Analyzing the Intel 80386DX
2-10
HP 103140
80386DX Preprocessor Interface
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