From 6cd83eb2dd7dea65db9a5431159b787af61a869f Mon Sep 17 00:00:00 2001
From: Cyrille Bagard <nocbos@gmail.com>
Date: Sun, 17 Jun 2018 17:54:57 +0200
Subject: Added a small file to play with syscalls.

---
 ARM/syscalls/Makefile     |  8 ++++++
 ARM/syscalls/syscalls.asm | 73 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 ARM/syscalls/Makefile
 create mode 100644 ARM/syscalls/syscalls.asm

diff --git a/ARM/syscalls/Makefile b/ARM/syscalls/Makefile
new file mode 100644
index 0000000..17a84e7
--- /dev/null
+++ b/ARM/syscalls/Makefile
@@ -0,0 +1,8 @@
+
+all: syscalls
+
+syscalls: syscalls.asm
+	grep .global syscalls.asm | cut -d ' ' -f 2 > keep.lst
+	arm-linux-gnueabi-as -o syscalls.o syscalls.asm
+	arm-linux-gnueabi-ld -s --retain-symbols-file=keep.lst -o syscalls syscalls.o
+	rm -f keep.lst
diff --git a/ARM/syscalls/syscalls.asm b/ARM/syscalls/syscalls.asm
new file mode 100644
index 0000000..1812e3b
--- /dev/null
+++ b/ARM/syscalls/syscalls.asm
@@ -0,0 +1,73 @@
+
+.data
+
+msg:
+    .ascii      "Hello, ARM!\n"
+
+len = . - msg
+
+no_arg_msg:
+    .ascii      "No command line argument...\n"
+
+no_arg_len = . - no_arg_msg
+
+got_arg_msg:
+    .ascii      "Got command line argument(s)...\n"
+
+got_arg_len = . - got_arg_msg
+
+.text
+
+.global do_syscalls
+
+do_syscalls:
+
+    /**
+     * syscall write(int fd, const void *buf, size_t count)
+     */
+
+    mov     %r0, $1     /* fd -> stdout */
+    ldr     %r1, =msg   /* buf -> msg */
+    ldr     %r2, =len   /* count -> len(msg) */
+    mov     %r7, $4     /* write is syscall #4 */
+    swi     $0          /* invoke syscall */
+
+    /**
+     * syscall write(int fd, const void *buf, size_t count)
+     */
+
+    mov     %r0, $2     /* fd -> stderr */
+    mov     %r7, $4     /* write is syscall #4 */
+
+    ldr     %r3, [sp]   /* argc */
+    cmp     %r3, $1
+
+    beq     no_arg
+
+    ldr     %r1, =got_arg_msg   /* buf -> msg */
+    ldr     %r2, =got_arg_len   /* count -> len(msg) */
+
+    b       process_arg
+
+no_arg:
+
+    ldr     %r1, =no_arg_msg   /* buf -> msg */
+    ldr     %r2, =no_arg_len   /* count -> len(msg) */
+
+process_arg:
+
+    swi     $0          /* invoke syscall */
+
+    /**
+     * syscall exit(int status)
+     */
+
+    mov     %r0, $123     /* status -> 0 */
+    mov     %r7, $1     /* exit is syscall #1 */
+    swi     $0          /* invoke syscall */
+
+.global _start
+
+_start:
+
+    bl      do_syscalls
-- 
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