start of examples

section_1/funcs/README3.md

@@ -0,0 +1,9 @@
+# Section 1 / Examples of calling common C runtime functions
+
+This chapter gives examples of calling common C runtime functions from
+assembly language.
+
+## Low level file operations
+
+The following example shows `open()`, `close()`, `read()`, `write()`
+and `lseek()`.

section_1/funcs/file_ops.s

@@ -0,0 +1,70 @@
+/*  Perry Kivolowitz
+    Example of file operations.
+*/
+        .text
+        .global main
+        .align  2
+
+/*  This program will
+    * open() a file in the current directory,
+    * write() some text to it, 
+    * seek back to the beginning of the file,
+    * read() each line, printing it
+    * close() the file
+*/
+
+retval  .req    w27
+fd 		.req	w28
+
+main:   stp     x29, x30, [sp, -16]!
+        stp     x27, x28, [sp, -16]!
+        bl      open_file
+        cbz     x0, 1f
+        // If we get here, the open has failed. Use perror() to
+        // print a meaningful error and exit.
+        bl      open_fail
+        b       99f
+
+1:		// When we get here, the file is open.
+
+		// When we get here, we are done. Close the file.
+		mov		w0, fd
+		bl 		close
+		mov 	retval, wzr
+
+99:     ldp     x27, x28, [sp], 16
+        ldp     x29, x30, [sp], 16
+        mov     w0, retval
+        ret
+
+/*	This function attempts to open a file for both reading and
+	writing. Return values will be checked to ensure the file is
+	opened. If successful, 0 is returned (and the file descriptor
+	is squirreled away in register "fd".
+*/
+
+open_file:
+       stp      x29, x30, [sp, -16]!
+       ldp      x29, x30, [sp], 16
+	   mov      x0, 1
+       ret
+
+
+/*  This function uses perror to print a meaningful error
+    message in the event of an open failure.
+*/
+
+open_fail:
+        stp     x29, x30, [sp, -16]!
+		ldr		x0, =fname
+		bl 		perror
+		mov		retval, 1
+        ldp     x29, x30, [sp], 16
+        ret
+
+
+		.data
+prog:	.asciz	"file_ops"
+fname:	.asciz	"test.txt"
+
+		.end