If you disagree about any of these choices, your count will come out different. For example, the operand-size prefix ( 0圆6) turns integer instructions operating on 32-bit registers into the equivalent instruction operating on their lower 16-bit halves, but unlike with the REP or LOCK prefixes, XED does not count these as separate instruction classes. But it doesn’t handle all prefixes this way in all contexts. The prefix thing crops up elsewhere: is say MOVSD (copy a single 32-bit word) a different “instruction class” from REP MOVSD (block copy several 32-bit words)? XED says yes. An atomic add does very different things from a regular add. And in fact, for the purposes of execution, so do current x86s.
Many assembly programmers would consider LOCK a prefix and LOCK ADD an addition with said prefix, not a distinct instruction, but XED disagrees.
For example, as per XED, ADD and LOCK ADD are different “instruction classes”. Well, it depends on what you wanted to count. According to Intel’s XED, as of this writing, there are 1503 defined x86 instructions (“iclasses” in XED lingo), from AAA to XTEST (this includes AMD-specific extensions too, by the way). To not leave you hanging: Intel has an official x86 encoder/decoder library called XED. It depends on how you count, and the details are interesting (to me anyway). One write operation, which occurs in the background.It’s surprisingly hard to give a good answer (the question was raised in this article).
The instruction execution cycle consists of: The above-mentioned 15 steps are required to accomplish the task defined by the instruction: mov BYTE PTR ds:, 45h. The data byte 45h enters into the DSM location: 03010h. The CM sends data write signal to the memory chip. The contents of TRB8 (45) is placed on the DBUS.ġ5. The resultant address is asserted on the ABUS.ġ4. The content of ds, bx and TRA8 are added together to get the destination memory address as: 03010h. The data byte 45h comes from the CSM and then enters into the TRB8 of U7.ġ3. The CM increments the PC, which is now 01003h. The data 10h (8-bit displacement) enters into the DAR from where it enters into TRA8 of U7.ġ0. The CM increments the PC, which is now 01002h. Addressing Mode is determined as ‘Immediate’, which means that the last information byte (45) is the data for the destination.ħ. Therefore, two more read operations must be carried out to bring the remaining two byes (10 45) information from the memory. The opcodes ‘C6 47’ are jointly decoded and the CM extracts the following information: The opcode 47 (content of the memory location 01001h) enters into opcode decoder. The CM increments the PC, which is now 01001h. The opcode is interpreted and the CM understands that current instruction has an opcode of two bytes long.Ĥ. The opcode C6 (content of CSM memory location 01000h) enters into the opcode decoder. The CM (Control Matrix, U4) sends the RD/-signal to the memory chip.ģ.
Opcode sheet for 8086 pdf Pc#
The PC (Program Counter, U1) asserts the address 01000h on the address bus.Ģ. Figure-2.45: Explaining Fetch-Execution Mechanism of ‘mov BYTE PTR ds:, 45h’ Instructionġ.
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