LODS

LODS loads the element at [RSI] into AL, AX, EAX or RAX and moves RSI to the next one, up or down as the direction flag says. It is the reading half of a hand-written string loop: lodsb, some work on AL, then stosb to store the result at [RDI], one byte each, which keeps such code short. Compiled code reads bytes with MOVZX instead: GCC 14 at -O2 compiles a byte-by-byte copy loop to movzx ecx, byte ptr [rsi + rax] and a byte MOV, stepping a single index.

The register it writes#

LODSD writes EAX and therefore clears bits 63:32 of RAX, like any 32-bit write. LODSB and LODSW replace only AL or AX and keep the rest of RAX, so a loop that uses all of RAX afterwards must clear it first, or widen AL with MOVZX.

REP LODS#

Both manuals accept a REP prefix, but each element overwrites the one before, so REP LODS leaves only the last element in rAX, with RSI past the end and RCX at 0; with RCX = 0 it leaves rAX alone. Zen 5 ran it exactly so, and the same with F2 for a count of 3. It does little that a single load and an ADD to RSI would not.

The source segment#

The source may take a segment prefix. In 64-bit mode only FS and GS add a base, so lods eax, dword ptr fs:[rsi] reads RSI bytes past the base of FS, which the psABI makes the thread pointer (see thread-local storage). STOS, its partner, cannot be redirected: it always writes through ES:RDI.

Speed#

The instruction table that comes with AMD’s Zen 5 guide lists LODSB, like the other string instructions, as microcoded, where a MOVZX from memory is a single macro-op. The direction flag, the resumption after a fault and the 67 prefix work as on the MOVS page.

Forms

Latency and throughput

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

step = -(OSIZE // 8) if df else OSIZE // 8
rAX = load(aSI, OSIZE)           # EAX: bits 63:32 cleared
aSI = low(aSI + step, ASIZE)

step = -(OSIZE // 8) if df else OSIZE // 8
s = aSI
for i in range(aCX):
    rAX = load(s, OSIZE)         # the last element stays; with aCX 0,
    s = low(s + step, ASIZE)     # rAX is left alone
aSI = s
aCX = 0

Operands

Flags

Attributes

What else XED records about the forms. How to read.

REP forms

repeated: count in rCX

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

More General-purpose instructions

• IMUL
• IN
• INC
• INS
• INT
• INT1
• INT3
• INTO
• IRET
• Jcc
• JCXZ
• JECXZ
• JMP
• JRCXZ
• LDS
• LEA
• LEAVE
• LES
• LFS
• LGS
• LOOP
• LOOPE
• LOOPNE
• LSS
• MOV
• MOVS
• MOVSX
• MOVSXD
• MOVZX
• MUL
• NEG
• NOP
• NOT
• OR
• OUT
• OUTS
• PAUSE
• POP
• POPA
• POPF

All 109 General-purpose instructions