问题
What are the "Op/En" formats listed in the Intel x86_64 reference manual? For example in the Add opcode I can take a guess at some such as "I" = Immediate, but is there a comprehensive list for these?
回答1:
The intro sections of Intel's vol.2 manual explain how to read each entry:
Section 3.1.1.4 Operand Encoding Column in the Instruction Summary Table
The “operand encoding” column is abbreviated as Op/En in the Instruction Summary table heading. Instruction operand encoding information is provided for each assembly instruction syntax using a letter to cross reference to a row entry in the operand encoding definition table that follows the instruction summary table. ...
NOTES
The letters in the Op/En column of an instruction apply ONLY to the encoding definition table immediately following the instruction summary table.
...
So they're just keys to the table that goes with each instruction. And no, Intel doesn't seem to document what they stand for, but it's pretty straightforward.
(Related: How to determine if ModR/M is needed through Opcodes?)
Yes, the I stands for Immediate.
Explicit operands encoded with the ModRM byte are M or R for the r/m or reg fields.
V is the field encoded by VEX.vvvv or EVEX, giving such instructions a 3rd operand for non-destructive operations, or for 3-operand instructions like FMA.
vpblendvb xmm1, xmm2, xmm3/m128, xmm4 encodes the 4th operand with an immediate byte, and uses RVMR in to Op/En column. See also What kind of address instruction does the x86 cpu have? - I think that might be the only x86 instruction with 4 separate explicitly-encoded operands, outside of AVX512's 3 + mask register.
vextractf128 and the AVX512 versions of it use A through D for rows of the table because the only distinguishing feature is the "tuple type" for scaled short-displacements and masking. It would just be weird if they used MRI / MRI2 / MRI4 / etc.
(rep) movs uses ZO : all operands are implicit (DF, RSI, RDI, and the memory pointed-to by them), so probably that stands for Zero Operands. (At least none that need to be encoded).
cdq uses the same ZO so yeah it's probably "zero (explicit) operands"
x86 only has a few ways to specify explicit operands.
来源：https://stackoverflow.com/questions/57440527/x86-64-opcode-encoding-formats-in-the-intel-manual