feat: we now read the passed directory in argv[1]; learned about aliasing immediate values; figured out how the syscall return values work; better error messages

main.S

@@ -1,27 +1,48 @@
 .global _start
 
-exit:
+exit: @ r0 should have the status code to exit with
-    @ Exit the program with a status code of 0
+    mov r7, #SYS_EXIT @ Move the exit syscall number to r7 (which is the register svc reads to know which syscall to execute)
-    @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: @ r1 => buffer to print. r2 => how many bytes to print. clobbers r0
     @ print to stdout
     mov r0, #1 @ Use STDOUT fd as the write fd
-    mov r7, #4 @ call write(2)
+    mov r7, #SYS_WRITE @ call write(2)
     svc 0
     bx lr @ return to caller
 
-fail:
+open_fail:
-    @ push the result of open() onto the stack before calling print()
+    push {r0} @ 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
+
+dir_read_fail:
+    push {r0} @ save the result of open() because print() clobbers r0
+    ldr r1, =DIR_READ_FAIL
+    mov r2, #29
+    bl print
+    pop {r0}
+
+    bl exit
+
+fd_not_dir:
+    ldr r1, =FD_NOT_DIR
+    mov r2, #24
+    bl print
+
+    mov r0, #1
+    bl exit
+
+fd_not_found:
+    ldr r1, =FD_NOT_FOUND
+    mov r2, #58
+    bl print
+
+    mov r0, #1
     bl exit
 
 open_and_read_file:
@@ -29,22 +50,22 @@ open_and_read_file:
     ldr r0, =FILE @ filepath
     mov r1, #0 @ flags
     mov r2, #0 @ mode_t
-    mov r7, #5
+    mov r7, #SYS_OPEN
     svc 0
 
     cmp r0, #0
-    blt fail
+    blt open_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
+    mov r7, #SYS_READ
     svc 0
 
     @ print the contents of the buffer
     ldr r1, =buffer
-    mov r2, #50
+    mov r2, #50 @ FIXME: hardcoded badness
     bl print
 
     ldr r1, =NEWLINE
@@ -54,7 +75,7 @@ open_and_read_file:
     mov r0, #0
     bl exit
 
-strlen:
+strlen: @ r0 holds the string length. r1 should be a pointer to the start of the string.
     mov r0, #0
 strlen_loop:
     ldrb r2, [r1, r0]
@@ -90,27 +111,37 @@ direntry_loop:
 
     b direntry_loop
 
-@ TODO: read args
-@ TODO: if no arg passed, use current directory.
 _start:
+    ldr r0, [sp]
+    cmp r0, #1
+    ldrgt r0, [sp, #8]
+    ldrle r0, =FALLBACK_DIR
+
     @ open file
-    ldr r0, =DIR @ filepath
+    @ldr r0, =FALLBACK_DIR @ filepath
     mov r1, #0 @ flags
     mov r2, #0 @ mode_t
-    mov r7, #5
+    mov r7, #SYS_OPEN
     svc 0
 
+    cmp r0, #ENOENT
+    beq fd_not_found
     cmp r0, #0
-    blt fail
+    blt open_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)
+    mov r7, #SYS_GETDENTS
     svc 0
 
+    cmp r0, #ENOTDIR
+    beq fd_not_dir
+    cmp r0, #0
+    blt dir_read_fail
+
     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
@@ -120,11 +151,24 @@ _start:
     bl exit
 
 .data
-    FILE: .asciz "/home/hisham/asm.txt"
+    FILE: .asciz "/home/illusion/asm.txt"
-    DIR: .asciz "/home/hisham"
+    FALLBACK_DIR: .asciz "."
-    OPEN_FAIL: .ascii "Failed to open file descriptor\n"
+    OPEN_FAIL: .ascii "Failed to open file descriptor"
+    DIR_READ_FAIL: .ascii "Failed to read the directory\n"
+    FD_NOT_DIR: .ascii "File is not a directory\n"
+    FD_NOT_FOUND: .ascii "Failed to open file descriptor: No such file or directory\n"
     NEWLINE: .ascii "\n"
 
+.equ SYS_EXIT,     0x01
+.equ SYS_READ,     0x03
+.equ SYS_WRITE,    0x04
+.equ SYS_OPEN,     0x05
+.equ SYS_FSTAT,    0x6C
+.equ SYS_GETDENTS, 0x8D
+
+.equ ENOENT,       0xFFFFFFFE
+.equ ENOTDIR,      0xFFFFFFEC
+
 .bss
 buffer:
     .skip 16384