Line Filters in Assembly Language

Here’s the translation of the line filter example from Go to Assembly Language:

Assembly Language doesn’t have built-in high-level constructs like string manipulation or file I/O. Therefore, we’ll implement a simplified version that reads input character by character, converts lowercase letters to uppercase, and writes the result to stdout. This example uses x86 assembly for Linux.

section .data
    error_msg db "Error reading input", 10
    error_len equ $ - error_msg

section .bss
    buffer resb 1

section .text
    global _start

_start:
    ; Main loop to read and process input
.read_loop:
    ; Read a single character
    mov eax, 3          ; sys_read
    mov ebx, 0          ; stdin
    mov ecx, buffer     ; buffer to store char
    mov edx, 1          ; read 1 byte
    int 0x80

    ; Check for end of file or error
    cmp eax, 0
    jl .error           ; jump if error (eax < 0)
    je .exit            ; jump if EOF (eax == 0)

    ; Convert lowercase to uppercase
    mov al, [buffer]
    cmp al, 'a'
    jl .write
    cmp al, 'z'
    jg .write
    sub al, 32          ; convert to uppercase

.write:
    ; Write the character
    mov [buffer], al
    mov eax, 4          ; sys_write
    mov ebx, 1          ; stdout
    mov ecx, buffer     ; buffer to write
    mov edx, 1          ; write 1 byte
    int 0x80

    jmp .read_loop

.error:
    ; Print error message
    mov eax, 4          ; sys_write
    mov ebx, 2          ; stderr
    mov ecx, error_msg
    mov edx, error_len
    int 0x80

.exit:
    ; Exit the program
    mov eax, 1          ; sys_exit
    xor ebx, ebx        ; return 0
    int 0x80

This assembly program implements a simple line filter that converts lowercase letters to uppercase. Here’s how it works:

1. We define a buffer to store a single character.

2. The main loop (.read_loop) reads one character at a time from stdin using the sys_read system call.

3. We check for errors or end of file (EOF) after each read operation.

4. If the character is a lowercase letter (between ‘a’ and ‘z’), we convert it to uppercase by subtracting 32 from its ASCII value.

5. We write the processed character to stdout using the sys_write system call.

6. The loop continues until we reach EOF or encounter an error.

7. If an error occurs, we print an error message to stderr.

8. Finally, we exit the program using the sys_exit system call.

To assemble and run this program:

$ nasm -f elf32 line_filter.asm
$ ld -m elf_i386 -o line_filter line_filter.o
$ echo 'hello' > /tmp/lines
$ echo 'filter' >> /tmp/lines
$ cat /tmp/lines | ./line_filter
HELLO
FILTER

This assembly language version is much more low-level than the original example. It doesn’t use any high-level constructs like buffered I/O or string manipulation functions. Instead, it operates on individual characters, which is more typical for assembly language programming.

Note that error handling in this version is simplified. In a real-world application, you might want to implement more robust error checking and handling.