Confused with CMPSB instruction
16-bit, assembly, bootloader, nasm, osdev
Solution
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.
Problem
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?