Assembly Language (NASM) problem My teacher gave me a problem about using the sa

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Assembly Language (NASM) problem

My teacher gave me a problem about using the same code from the previous assignment, which is read an input file, do ROT13 encoding for that file, and then output to a text file.

This time instead of input and output using int 80h, it should be replace by glibc functions, and other related changes should be made. Also print a character count. This problem requires character I/O, so putchar() and getchar() should be the functions to use (Lecture 15, slide 48). Notice that these functions use type int whereas a character is only one byte. Return values are always in EAX

This is my code for the last assignment:

section .bss
Buff resb 1

section .data

section .text
global _start

_start:
nop ; This no-op keeps the debugger happy

Read:   mov eax,3      ; Specify sys_read call
mov ebx,0      ; Specify File Descriptor 0: Standard Input
mov ecx,Buff   ; Pass address of the buffer to read to
mov edx,1      ; Tell sys_read to read one char from stdin
int 80h        ; Call sys_read

cmp eax,0;
je Exit;
cmp byte [Buff],41h ; check if below A in ASCII
jb Write;       ; if below then stdout
cmp byte [Buff],5Ah ; check if above Z in ASCII
ja lowercheck       ; if above then jump to check if from a-z
cmp byte [Buff],4Dh; check if from A-M
ja encodehigh; if not jump to encode for N-Z
jmp encodelow; jump to encode A-M

lowercheck:
    cmp byte [Buff],61h; check with a in ASCII
    jb Write; stdout if below
    cmp byte [Buff],7Ah; check with z in ASCII
    ja Write; stdout if above
    cmp byte [Buff],6Dh; check if from a-m
    ja encodehigh; if not jump to encode for n-z
    jmp encodelow; jump to encode for a-m

encodehigh:
    sub byte [Buff],0Dh; -13 to encode if from n-z
    jmp Write; stdout
encodelow:
    add byte [Buff],0Dh; +13 to encode if from a-m
    

Write: mov eax,4      ; Specify sys_write call
mov ebx,1      ; Specify File Descriptor 1: Standard output
mov ecx,Buff   ; Pass address of the character to write
mov edx,1      ; Pass number of chars to write
int 80h        ; Call sys_write...
jmp Read      ; ...then go to the beginning to get another character

Exit:   mov eax,1      ; Code for Exit Syscall
mov ebx,0      ; Return a code of zero to Linux
int 80H        ; Make kernel call to exit program

Explanation / Answer

section .data

section .text
global _start

_start:
nop ; This no-op keeps the debugger happy

Read:   mov eax,3      ; Specify sys_read call
mov ebx,0      ; Specify File Descriptor 0: Standard Input
mov ecx,Buff   ; Pass address of the buffer to read to
mov edx,1      ; Tell sys_read to read one char from stdin
int 80h        ; Call sys_read

cmp eax,0;
je Exit;
cmp byte [Buff],41h ; check if below A in ASCII
jb Write;       ; if below then stdout
cmp byte [Buff],5Ah ; check if above Z in ASCII
ja lowercheck       ; if above then jump to check if from a-z
cmp byte [Buff],4Dh; check if from A-M
ja encodehigh; if not jump to encode for N-Z
jmp encodelow; jump to encode A-M

lowercheck:
    cmp byte [Buff],61h; check with a in ASCII
    jb Write; stdout if below
    cmp byte [Buff],7Ah; check with z in ASCII
    ja Write; stdout if above
    cmp byte [Buff],6Dh; check if from a-m
    ja encodehigh; if not jump to encode for n-z
    jmp encodelow; jump to encode for a-m

encodehigh:
    sub byte [Buff],0Dh; -13 to encode if from n-z
    jmp Write; stdout
encodelow:
    add byte [Buff],0Dh; +13 to encode if from a-m
    

Write: mov eax,4      ; Specify sys_write call
mov ebx,1      ; Specify File Descriptor 1: Standard output
mov ecx,Buff   ; Pass address of the character to write
mov edx,1      ; Pass number of chars to write
int 80h        ; Call sys_write...
jmp Read      ; ...then go to the beginning to get another character

Exit:   mov eax,1      ; Code for Exit Syscall
mov ebx,0      ; Return a code of zero to Linux
int 80H        ; Make kernel call to exit program

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