x86 machine code format for register indirect mode
The instruction format for x86 family which is shown below sounds ambiguous.
Fields: opcode,direction,width mod,reg,r/m
Bits: 6 1 1 2 3 3
Assume the instruction is MOV AX, [BP]. No matter what is the opcode, we have the following numbers:
direction=1
width=1
For the second byte, the reg is 000 (code for AX) and mod should be 00 but there is no entry for bp in the table. So, the r/m is unknown.
If we assume mod is 11 and r/m is 101, then that is the machine code for an instruction like MOV AX, BP.
Any idea?
assembly x86 emu8086
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
add a comment |
The instruction format for x86 family which is shown below sounds ambiguous.
Fields: opcode,direction,width mod,reg,r/m
Bits: 6 1 1 2 3 3
Assume the instruction is MOV AX, [BP]. No matter what is the opcode, we have the following numbers:
direction=1
width=1
For the second byte, the reg is 000 (code for AX) and mod should be 00 but there is no entry for bp in the table. So, the r/m is unknown.
If we assume mod is 11 and r/m is 101, then that is the machine code for an instruction like MOV AX, BP.
Any idea?
assembly x86 emu8086
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
8
Mov ax,[bp]is shortened forMov ax,[bp+0x00], mod is "01", and rm "110" ( for (bp)+D8 )
– Tommylee2k
Nov 23 '18 at 10:55
add a comment |
The instruction format for x86 family which is shown below sounds ambiguous.
Fields: opcode,direction,width mod,reg,r/m
Bits: 6 1 1 2 3 3
Assume the instruction is MOV AX, [BP]. No matter what is the opcode, we have the following numbers:
direction=1
width=1
For the second byte, the reg is 000 (code for AX) and mod should be 00 but there is no entry for bp in the table. So, the r/m is unknown.
If we assume mod is 11 and r/m is 101, then that is the machine code for an instruction like MOV AX, BP.
Any idea?
assembly x86 emu8086
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
The instruction format for x86 family which is shown below sounds ambiguous.
Fields: opcode,direction,width mod,reg,r/m
Bits: 6 1 1 2 3 3
Assume the instruction is MOV AX, [BP]. No matter what is the opcode, we have the following numbers:
direction=1
width=1
For the second byte, the reg is 000 (code for AX) and mod should be 00 but there is no entry for bp in the table. So, the r/m is unknown.
If we assume mod is 11 and r/m is 101, then that is the machine code for an instruction like MOV AX, BP.
Any idea?
assembly x86 emu8086
assembly x86 emu8086
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
asked Nov 23 '18 at 10:22
mahmoodmahmood
7,5503090139
7,5503090139
8
Mov ax,[bp]is shortened forMov ax,[bp+0x00], mod is "01", and rm "110" ( for (bp)+D8 )
– Tommylee2k
Nov 23 '18 at 10:55
add a comment |
8
Mov ax,[bp]is shortened forMov ax,[bp+0x00], mod is "01", and rm "110" ( for (bp)+D8 )
– Tommylee2k
Nov 23 '18 at 10:55
8
8
Mov ax,[bp] is shortened for Mov ax,[bp+0x00], mod is "01", and rm "110" ( for (bp)+D8 )– Tommylee2k
Nov 23 '18 at 10:55
Mov ax,[bp] is shortened for Mov ax,[bp+0x00], mod is "01", and rm "110" ( for (bp)+D8 )– Tommylee2k
Nov 23 '18 at 10:55
add a comment |
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8
Mov ax,[bp]is shortened forMov ax,[bp+0x00], mod is "01", and rm "110" ( for (bp)+D8 )– Tommylee2k
Nov 23 '18 at 10:55