How to search for a file in a FAT12 system in Assembly -

currently, i've been following brokenthorn series on os dev, , i've run bit of problem. right now, part of journey through tutorial, i'm coding part loads in second stage bootloader, unfortunately, code crashes. here portion of code think troublesome:

code:

;browse root directory binary image      mov     ax, word [bpbrootentries]; load loop counter, bpbrootentries number of entries in fat table      mov     di, 0x0000 ; because rep cmpsb compares string in es:di ds:si, , es holds 0x7e00 (the location of fat table), decided set di 0x0000      mov     cx, 0x000b; eleven character name                 lea     si, [imagename] ;set si memory location of imagename ds:si points imagename            .loop:       rep  cmpsb            jz     load_fat       add     di, 32                            ; queue next directory entry       dec ax       cmp ax, 0x0  jne .loop   jmp     failure 

this portion of code looks file in fat table. however, not able find it, , crashes. in code, imagename variable value "krnldr sys" in it. in floppy drive, have file called "krnldr sys" in floppy drive (with spaces, not "krnldr.sys"). great if offer advice.

note: i'm runnning 64-bit windows 7 pc

update

after helpful comments, have updated code:

mov     ax, word [bpbrootentries]             ; load loop counter  mov     di, 0x0000                            ; locate first root entry  mov     cx, 0x000b                            ; eleven character name                 lea     si, [imagename]                         ; image name find              .loop:       push di       push si       repe  cmpsb          pop di       pop si       jz     load_fat        add     di, 32                            ; queue next directory entry        dec ax       or ax, ax       jne .loop        jmp     failure 

unfortunately, os still not able find file.

update 2

here code i've used load root directory table:

     load_root:   ; compute size of root directory , store in "cx"        xor si, si        mov     ax, 0x0020                           ; 32 byte directory entry       mul     word [bpbrootentries]                ; total size of directory       div     word [bpbbytespersector]             ; sectors used directory       xchg    ax, cx   ; compute location of root directory , store in "ax"        mov     al, byte [bpbnumberoffats]            ; number of fats       mul     word [bpbsectorsperfat]               ; sectors used fats       add     ax, word [bpbreservedsectors]         ; adjust bootsector       mov     word [datasector], ax                 ; base of root directory       add     word [datasector], cx   ; read root directory memory (7c00:0200)       mov   dx, 0x7e00      mov   es, dx      mov     bx, 0x0                             ; copy root dir above bootcode      call    readsectors 

thanks!

the rep on cmpsb repeat cmpsb cx times regardless , zf set result of last comparison made.

• you need repe here

even repe, repe cmpsb modify both di , si each point byte after last byte compared (let's assume df set up) hence adding 32 di certainly won't point next fat entry.

• you need push both di , si before repe cmpsb , pop them after (which won't affect flags)

that way, neither di nor si appears changed , adding of 32 valid.

by way of finesse, or ax,ax set zf same cmp ax,0x0 smaller, faster instruction.