I believe it is
rep stosd
For repeated store string DWORD (32 bits).
(On CPUs with the ERMSB feature, rep stosb
is efficient so you don't need to imul eax, 0x01010101
to set up for rep stosd
if you have a byte value instead of a dword pattern.)
You can find the stos_
instructions in the Intel instruction set reference. They use the suffixes b
for 1-byte, w
for 2-byte, d
for 4-byte, and q
for 8-byte string operations.
There's a separate manual entry for rep
/repe
/repne
which documents every instruction it's valid with as an actual repeat prefix.
(But note that only rep stos_
and rep movs_
have Fast Strings microcode; all the conditional repe/ne cmps/scas instructions are slow on modern CPUs, only going 1 count per 1 or even 2 clocks.)