I have been looking at this code and I'm confused about the rep cmpsb line.
.LOOP:
push cx
mov cx, 0x000B ; eleven character name
mov si, ImageName ; image name to find
push di
rep cmpsb ; test for entry match
pop di
je LOAD_FAT
pop cx
add di, 0x0020 ; queue next directory entry
loop .LOOP
jmp FAILURE
I understand that it repeats cmpsb cx times but how does this compare the two strings? Say for example was comparing "Hey\0" and "hey\0" and this loop was comparing 4 character strings. The first characters are different and the EFlags register would be set accordingly. However, the cmpsb instruction is repeated and the next characters would be the same. I may be misunderstanding how cmpsb works but it looks like this loop does not correctly compare two strings. Does this loop in fact work?
The reason REP works is because rep has the same encoding as REPE (F3h). In principle REPE is the right thing to use here, but depending on your assembler it might just take REP as correct.
So in reality you have a REPE cmpsb there, it's just that your (dis)assembler doesn't really know.