A constant argument of 3, implicit in the opcode, and not represented elsewhere in the instruction. If it is a memory address, the address is computed from a segment register and any of the following values: I wanted to focus on integer opcodes in this map, as floating-point would be exceedingly rare in production code. I wouldn't expect to see this in code as the "POP CS" instruction is particularly useless and wanted to treat its appearance as an error condition.
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The operand is either a general-purpose register or a memory address.
Disassembly by hand Building the map lines of Python. GRP2 E b 1. This is sehet HTML-ized version of the opcode map for the processor. The one remaining complexity involves "group" opcodes, such as All the preceeding remarks about opcode 84 apply equally here.
Note that arguments may be specified in both the opcode map and the opcode extensions table e. I want to use this map to build a disassembler, not a simulated processor, and the extra arguments would only be burdensome.
Normally, however, the arguments from the opcode map are used. I wanted to focus on integer opcodes in this map, as floating-point would be exceedingly rare in production code.
The operand value is encoded in subsequent bytes of the instruction.
This restriction is not shared with other opcodes with "E"-addressed arguments, and not reflected in the map. I wanted as simple a map as possible, and, to that end, this map contains some lacunae: The instruction contains a relative offset to be added to the address of the subsequent instruction.
Other special symbols can be looked up in the " Special Argument Codes " table. Other values are illegal. Arguments are either a pair of letters - the first in upper case, the second in lower case - or a special symbol. In addition to the information that was removed, this map contains two known errors. Both operands are of type "v", so both are WORDs.
GRP2 E v 1. SI turns out to represent as one might expect the bit SI register, so opcode 4E simply decrements this register by 1. Yes, with nearly 30 years hindsight, there probably shouldn't be an entire opcode devoted to this operation.
However, if you see something that doesn't look right, please contact me. Unusual in that arguments of this type are suppressed in ASM output when they have the default value of 10 0xA. If you're interested in reading more about the disassembler, the following posts might be worth a look:.
As far as I know, this opcode map is, modulo the lacunae and errata mentioned above, correct. A constant argument of 3, implicit in the opcode, and not represented elsewhere in the instruction.
shewt To disassemble "group" opcodes, consult the " Opcode Extensions " table for any entry in the opcode map with a mneumonic of the form GRP.
This map was constructed by taking a map for a more recent x86 processor and removing information irrelevant to the much earlier processor.
To use the map, find the cell in the row labelled with the opcode's most significant 4 bits, and the column labelled with the opcode's least significant 4 bits. A4 through A7, 9C, 9D which correspond to instructions which take no arguments when written as assembly code e. This distinction only affects dis assembly, since the order of operands is irrelevant to TEST's function. A plain-text version - easily parsable by software - is also available.
The map is split in half; columns appear in the first partwhile columns 8-F appear in the second. If it is a memory address, the address is computed from a segment register and any of the following values: