initial commit

main.S

@@ -0,0 +1,132 @@
+.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