From 7a02913e36503a13514edc4986bd455de726c541 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Sun, 2 Jan 2022 16:19:33 +0100
Subject: [PATCH] cat start

---
 build.sh    |  8 +++--
 src/cat.asm | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/wc.asm  |  1 -
 3 files changed, 99 insertions(+), 3 deletions(-)
 create mode 100644 src/cat.asm

diff --git a/build.sh b/build.sh
index b2d636e..fc49214 100644
--- a/build.sh
+++ b/build.sh
@@ -11,7 +11,7 @@ assemble () {
     echo "Assembling $PROGRAM_NAME"
   fi
 
-  nasm -g -F dwarf -f elf64 "$FILE" -o "./target/$PROGRAM_NAME.o" || return
+  nasm -g -F dwarf -f elf64 "$FILE" -o "./target/$PROGRAM_NAME.o"
 }
 
 build () {
@@ -29,7 +29,11 @@ build () {
       return
     fi
 
-    assemble "$FILE" "$PROGRAM_NAME"
+
+    if ! assemble "$FILE" "$PROGRAM_NAME"; then
+        echo "Errors assembling $PROGRAM_NAME"
+        return
+    fi
 
     if [ "$QUIET" = "false" ]; then
       echo "Linking $PROGRAM_NAME"
diff --git a/src/cat.asm b/src/cat.asm
new file mode 100644
index 0000000..77cc96d
--- /dev/null
+++ b/src/cat.asm
@@ -0,0 +1,93 @@
+          extern     open_file_arg
+
+          global     _start
+
+IO_BUF_SIZE  EQU 1024
+STDIN_FD     EQU 0
+STDOUT_FD    EQU 1
+
+          section   .data
+file_not_found_msg:
+          db        'File not found', 10, 15
+failed_to_read_msg:
+          db        'Failed to read', 10, 15
+io_buf:   times IO_BUF_SIZE db 0
+newline:  db        10, 1
+
+          section   .text
+_start:
+          pop       rax                         ; argc
+
+          cmp       rax, 1
+          je        stdin_init                  ; if we don't have any arguments, read from stdin
+
+          ; we do have at least one argument, open the file
+          pop       rbx                         ; program name
+          pop       rax                         ; filename
+          call      open_file_arg
+          ; test whether there was an error
+          cmp       rax, 0
+          jl        file_not_found
+
+          mov       rdi, rax
+          jmp       init
+
+stdin_init:
+          mov       rdi, STDIN_FD
+
+init:
+          ; the input fd is in rdi at this point
+          xor       r13, r13
+process:
+          ; read in from the file
+          mov       rax, 0
+          mov       rsi, io_buf
+          mov       rdx, IO_BUF_SIZE
+          syscall
+
+          ; test whether it is finished
+          cmp       rax, 0
+          jz        finish
+
+          ; test whether there was an error
+          cmp       rax, 0
+          jl        failed_to_read
+
+          ; write to the file
+          mov       rdx, rax
+          mov       rax, 1
+          mov       rsi, io_buf
+          mov       rdi, STDOUT_FD
+          syscall
+
+          jmp       process
+
+finish:
+          ; write a trailing newline
+          mov       rax, 1
+          mov       rdx, 1
+          mov       rsi, newline
+          mov       rdi, STDOUT_FD
+          syscall
+
+exit_success:
+          xor       rdi, rdi
+exit:
+          mov       rax, 60
+          syscall
+
+failed_to_read:
+          mov       rax, 1                      ; write
+          mov       rdi, 2                      ; stderr
+          mov       rsi, failed_to_read_msg     ; buf
+          mov       rdx, 15                     ; len
+          jmp       exit
+
+file_not_found:
+          mov       rax, 1                     ; write
+          mov       rdi, 2                     ; stderr
+          mov       rsi, file_not_found_msg    ; buf
+          mov       rdx, 15                    ; len
+          syscall
+          mov       rdi, 1
+          jmp       exit
diff --git a/src/wc.asm b/src/wc.asm
index 078ece1..7965993 100644
--- a/src/wc.asm
+++ b/src/wc.asm
@@ -1,4 +1,3 @@
-          extern     itoa
           extern     println_num
           extern     open_file_arg