assembly - Having trouble encrypting and decrypting with cmp -

i'm having issue cmp. need incorporate 3 tables encrypt , decrypt. can play code either n_tab, l_tab, or a_tab can't seem make sense of incorporating three. appreciated. thank

.model small  .stack 100h  .data       prompt1 db "enter message here: $"      prompt2 db "encrypted: $"     prompt3 db "decrypted: $"      a_tab db "zyxwvutsrqponmlkjihgfedcba"        buff  db 50 dup(0)         .code      main:     mov ax, @data     mov ds, ax                      ; set addressability     mov si, offset buff             ; put buff      lea dx, prompt1                 ; load , print string prompt     mov ah, 9     int 21h     mov si, di      mov ah, 3fh     mov cx, 50     int 21h     mov si, di          mov bx, offset a_tab     mov si, di      l1: lodsb     push bx     mov ah, 6     mov dl, 0ffh     int 21h     pop bx     mov dl, al     mov al, 0     sub al, 'a'      xlat     loop l1      lea dx, prompt2                 ; load , print string prompt     mov ah, 9     int 21h     mov si, di      mov ah, 4ch     int 21h  end main 

there few problems see i'll describe. sorry don't have dos or dos emulator or windows set test this, i'm reading through code. there may other issues well, these significant ones:

    mov ah, 3fh     mov bx, 0     mov cx, 50     mov dx, offset buff         ; put buff     int 21h 

the start of segment sets file i/o read file handle 0 (standard input). when int 21h returns, actual number of bytes read in ax, seems ignored, ultimately.

then loop starts...

l1:     mov dl, al     mov bx, offset a_tab 

you're beginning encryption loop. you've moved low byte of number of bytes read user dl , loaded a_tab address bx set xlat expects table address in ds:bx , index table value in al, loading result al.

    xlat 

so here have trouble. in al when xlat first performed? had number of bytes read in standard input, not intended. assume mov dl, al somehow intended next character out of buffer, need al before xlat call. means have read out of buffer. whither buffer index or advancing buffer pointer in loop? doesn't appear setup.

moving on...

    mov ah, 6     mov dl, 0ffh     int 21h 

this outputs character that's in al terminal. whatever retrieved a_tab using number of bytes read terminal index should seeing first character output.

remember instruction @ l1: above did mov dl, al? seems have no effect since dl hasn't been used yet , being overwritten 0ffh above dos interrupt.

    lea dx, prompt2               ; load , print string prompt     loop l1 

this says you're loading , printing prompt string, seems that's you're starting (with lea) loop l1. assume loop instruction belongs before lea instruction here. loop decrement counter in cx , jump l1 if result not zero. however, in cx? answer: uncertain because last dos interrupt executed output character may have altered cx since it's not guaranteed preserve registers. , if did last known value of cx mov cx, 50 set maximum buffer size (not actual buffer size, returned buffer read ignored code). interrupt might have obliterated cx.

i'm supposing trying index translation table input characters. have first decide whether you're doing upper case, lower case, or both input, have enough entries in output table, a_tab account that. need read in character, subtract off lowest possible character value (which 'a' if you're using upper case letters), use value index table.

you're seeing garbled output because there's no telling what's in al time execute xlat instruction each time through loop.

to make code work properly, here steps follow:

before loop

1. read input string buffer, doing fine, , save number of bytes read loop counter, cx. of x86 loop instructions use cx counter.
2. set spare register, si address of current buffered input character (set si buf start). si register ideal buffer indexing (look lods, stos, etc, instructions).

during loop

1. read byte input buffer al (you can use lodsb instruction automatically increment si next load)
2. if you're reading caps input, subtract 'a' character in al. if lower case, subtract 'a'. need zero-base index a_tab.
3. put address of a_tab bx
4. execute xlat instruction load translated result al.
5. save registers care onto stack using push; includes cx register current loop counter , si
6. use dos interrupt output character in al
7. restore registers saved in step 5 using pop
8. go around loop loop instruction decrement loop counter, cx.

these overall steps. may want other things around well, should on right track. should read documentation online special instructions such xlat , lodsb understand behavior. in addition, anytime call dos interrupt or function, has conventions how pass arguments, , how information might returned in registers. other registers not mentioned need might used interrupt or function without knowing, should save registers before doing such calls, using push example, , restore them after call using pop.