Prob 8feb2018

a.asm ####################################################################################################################################
bits 32
global start,r
extern exit,printf,maxBytes
import exit msvcrt.dll
import printf msvcrt.dll

segment data use32 class=data
    s dd 1234A678h,12345678h,1AC3B47Dh,0FEDC9876h
    lenS equ ($-s)/4
    r times lenS db 0
    d resd 1
    sum resd 1
    pf db "%Xh ",0
    pf2 db `\nThe sum (signed) sum of these bytes is %d\n`,0

segment code use32 class=code
start:
    ;maxBytes(s,lenS)
    push lenS
    push s
    call maxBytes
    add ESP,4*2
    ;this will return in eax the sum of the max bytes
    mov [sum],EAX

    CLD
    mov ESI,s
    mov EDI,0
    ; now print these bytes using the damn string of byte ranks
    ; that was built using the function from b.asm
    printBytes:
        LODSD ; EAX = ESI , ESI+=4
        mov [d],EAX ; put in [d] the current dword
        mov EBX,d ; put its offset in EBX to use the amazing XLAT
        mov AL,4       ; because of little endian
        sub AL,[r+EDI] ; now we get the good position
        XLAT ; AL := [d+AL]
        movzx EAX,AL

        ;now print the byte extracted from the current dword
        push EAX
        push pf
        call [printf]
        add ESP,4*2

        inc EDI
        cmp EDI,lenS
        jb printBytes

    ;print(pf,sum)
    push dword[sum]
    push pf2
    call [printf]
    add ESP,4*2

    jmp $ ; keeps your compiled exe open
    push dword 0
    call [exit]


b.asm ##################################################################################################################################

bits 32
extern r
global maxBytes

segment data use32 class=data
    max resb 1 ; used to calculate the max byte
    d resd 1 ; local variable used by the function

segment code use32 class=code
    maxBytes:
    mov ESI, [ESP+4] ; s parameter
    mov ECX, [ESP+8] ; lenS parameter
    mov EBX,0 ;initialize sum
    mov EDI,r ;string from a.asm that will store the max bytes ranks
    CLD

    parseS:
    push ECX ; save ECX cuz we gonna need it l8r
    LODSD ; put in EAX the current dword
    mov [d],EAX
    mov ECX,0 ; this will be our byte index
    mov AL,1 ; initialize rank
    mov [max],CL ; initialize max byte
    findMax:
        mov DL,[d+ECX]
        cmp DL,[max]
        jbe next
        mov [max],DL
        mov AL,4 ; we have to do this shit bc little endian
        sub AL,CL ; now we have the rank in AL
        next:
        inc ECX
        cmp ECX,4 ; check if we interated over all the 4 bytes of the dword
        jb findMax

    STOSB ; in AL we have the rank of the max byte
    movsx EAX,byte[max] ; in [max] we have the max byte
    add EBX,EAX ; add the signed byte to the sum
    pop ECX  ; restore the saved ECX
    loop parseS

    mov EAX,EBX ; ret the signed sum in EAX
    ret