cat: add general cat framework (special cases and output processing soon)

1 files changed, 92 insertions, 5 deletions

diff --git a/cat.s b/cat.s
index 79d9f30..8a047e7 100644
--- a/cat.s
+++ b/cat.s
@@ -1,16 +1,103 @@
     global _start
     section .text
-_start:
+process_fd:
+    ; rax = fd
+
+    ; read(rax, buf, bufSize)
+    mov rbx, rax
+    mov rdi, rax
+    mov rsi, buf
+    mov rdx, bufSize
+    mov rax, 0
+    syscall
+
+    ; check if stuff was read
+    cmp rax, 1
+    jl .ret
+
+    ; write(1, buf, rax)
+    mov rdi, 1
+    mov rsi, buf
+    mov rdx, rax
     mov rax, 1
+    syscall
+
+    ; continue loop
+    mov rax, rbx
+    jmp process_fd
+.ret:
+    ; return rax
+    mov rax, rbx
+    ret
+
+_start:
+    ; r12 = argc
+    mov rbp, rsp
+    mov r12, [rbp]
+    cmp r12, 2
+    jl .stdin
+
+    add rbp, 8 * 2 ; jump past 1st arg
+
+.loop:
+    ; if no more entries; exit
+    cmp r12, 2
+    jl exit
+
+    ; arguments starting with "-" are special cases
+    mov r13, [rbp]
+    cmp r13, 0x2d ;'-'
+    je .special
+
+    ; open(rbp, 0, 0)
+    mov rax, 2
+    mov rdi, [rbp]
+    mov rsi, 0
+    mov rdx, 0
+    syscall
+
+    ; make sure fd exists
+    cmp rax, 1
+    jl .error
+    ; print file
+    call process_fd
+    ; returns input
+    ; close(rax)
+    mov rdi, rax
+    mov rax, 3
+    syscall
+
+    ; prepare registers for next loop
+    add rbp, 8
+    sub r12, 1
+    jmp .loop
+
+    ; unreachable. insurance
+    jmp exit
+
+.special:
+    ;special case handler (TODO)
+    ; stdin passthrough
+.stdin:
+    mov rax, 0
+    call process_fd
+    jmp .loop
+
+.error:
+    ;exit(1)
+    ; TODO: process RAX to print meaningful message
+    mov rax, 60
     mov rdi, 1
-    mov rsi, msg
-    mov rdx, msgSize
     syscall
 
+exit:
+    ;exit(0)
     mov rax, 60
     mov rdi, 0
     syscall
 
     section .data
-msg: db "Hello, world!", 10
-msgSize: EQU $ - msg
+
+    section .bss
+bufSize: equ 65536
+buf: resb bufSize