SDM entry reader: DEC & ROL

Opcode	Instruction	Op/En	64-bit	Description
FE /1	DEC r/m8	M	Valid	Decrement r/m8 by 1
FF /1	DEC r/m16	M	Valid	Decrement r/m16 by 1
FF /1	DEC r/m32	M	Valid	Decrement r/m32 by 1
REX.W + FF /1	DEC r/m64	M	Valid	Decrement r/m64 by 1

Flag	Status	Detail
CF	not affected	DEC deliberately does not touch CF. This distinguishes it from SUB r/m, 1 which would set CF on underflow.
OF	modified	Set if signed overflow occurred (DEST was INT_MIN before the decrement).
SF	modified	Set to the MSB of the result.
ZF	modified	Set if result == 0.
AF	modified	Set if borrow from bit 3 to bit 4.
PF	modified	Set if low byte of result has even parity.

Opcode / instruction table

Opcode	Instruction	Op/En	64-bit	Description
D0 /0	ROL r/m8, 1	M1	Valid	Rotate 8 bits r/m8 left once
D2 /0	ROL r/m8, CL	MC	Valid	Rotate 8 bits r/m8 left CL times
C0 /0 ib	ROL r/m8, imm8	MI	Valid	Rotate 8 bits r/m8 left imm8 times
REX.W + D1 /0	ROL r/m64, 1	M1	Valid	Rotate 64 bits r/m64 left once
REX.W + D3 /0	ROL r/m64, CL	MC	Valid	Rotate 64 bits r/m64 left CL times

Operation pseudocode — click a line to annotate it

1COUNT ← COUNT MOD SIZE;

2WHILE (COUNT ≠ 0) {

CF ← MSB(DEST);3

DEST ← (DEST << 1) + CF;4

COUNT ← COUNT – 1;5

6}

7IF (COUNT == 1) {

OF ← MSB(DEST) XOR CF;8

} ELSE IF (COUNT == 0) {9

/* OF undefined */10

11}

Click any line above to see an explanation.

Flags affected

Flag	Status	Detail
CF	modified	After each rotation, CF holds the bit that wrapped around from MSB to bit 0 position — it's always the last bit rotated out of the top.
OF	undefined if count > 1	Defined only for count=1 (OF = MSB XOR CF). For count > 1, the SDM says OF is undefined — the hardware may set it to anything. Do not rely on it.
SF	not affected	ROL is a bit manipulation op, not arithmetic. SF is not updated.
ZF	not affected	Not updated by rotate instructions.
AF	undefined	The SDM explicitly marks AF as undefined after ROL. Any value may appear.
PF	not affected	Not updated by rotate instructions.

Interactive SDM entry reader for DEC and ROL instructions, with annotated pseudocode and flags sections