ls-arm/main.S

.global _start

exit:
    @ Exit the program with a status code of 0
    @mov r0, #0 @ `exit` syscall requires the status code to be written to r0
    mov r7, #1 @ `exit` syscall's number is 1. Move it to r7 (which is the register svc reads to know which syscall to execute)
    svc 0 @ execute the syscall

print: @ r1 => buffer to print. r2 => how many bytes to print
    @ print to stdout
    mov r0, #1 @ Use STDOUT fd as the write fd
    mov r7, #4 @ call write(2)
    svc 0
    bx lr @ return to caller

fail:
    @ push the result of open() onto the stack before calling print()
    push {r0}
    ldr r1, =OPEN_FAIL
    mov r2, #31
    bl print
    pop {r0}

    @mov r0, #1
    bl exit

open_and_read_file:
    @ open file
    ldr r0, =FILE @ filepath
    mov r1, #0 @ flags
    mov r2, #0 @ mode_t
    mov r7, #5
    svc 0

    cmp r0, #0
    blt fail

    @ read the file contents
    @ r0 needs the fd, which it should already be since the call to open() set r0 to the fd.
    ldr r1, =buffer @ pointer to buffer to read into
    mov r2, #50 @ how many bytes to read
    mov r7, #3
    svc 0

    @ print the contents of the buffer
    ldr r1, =buffer
    mov r2, #50
    bl print

    ldr r1, =NEWLINE
    mov r2, #1
    bl print

    mov r0, #0
    bl exit

strlen:
    mov r0, #0
strlen_loop:
    ldrb r2, [r1, r0]
    cmp r2, #0
    bxle lr

    add r0, r0, #1
    b strlen_loop

direntry_loop:
    cmp r4, r8
    bxge lr

    push {lr}

    add r1, r5, #10 @ offset 10 bytes into the dirent struct to reach d_name.
    bl strlen
    mov r2, r0
    bl print

    ldr r1, =NEWLINE
    mov r2, #1
    bl print

    pop {lr}

    @ load the value (as 16-bits zero-extended as opposed to loading 32-bits and then extending that ourselves with UXTH)
    @ in the memory address of r5 offset by 8 bytes to get us the d_reclen
    ldrh r6, [r5, #8] 
    add r5, r5, r6 @ skip to the next dirent

    add r4, r4, r6 @ accumulate how many bytes we read

    b direntry_loop

@ TODO: read args
@ TODO: if no arg passed, use current directory.
_start:
    @ open file
    ldr r0, =DIR @ filepath
    mov r1, #0 @ flags
    mov r2, #0 @ mode_t
    mov r7, #5
    svc 0

    cmp r0, #0
    blt fail

    @ read the directory contents
    @ r0 needs the fd, which it should already be since the call to open() set r0 to the fd.
    ldr r1, =buffer @ pointer to linux_dirent structure
    @ TODO: we can allocate the required memory on the stack instead of in the bss section I think.
    mov r2, #16384 @ how many bytes to read
    mov r7, #0x8d @ getdents(2)
    svc 0

    mov r8, r0 @ store how many bytes the getdents syscall read, so we can use that to exit the direntry loop
    mov r4, #0 @ initialize r4 to 0
    ldr r5, =buffer
    bl direntry_loop

    mov r0, #0
    bl exit

.data
    FILE: .asciz "/home/hisham/asm.txt"
    DIR: .asciz "/home/hisham"
    OPEN_FAIL: .ascii "Failed to open file descriptor\n"
    NEWLINE: .ascii "\n"

.bss
buffer:
    .skip 16384
name:
    .skip 256