lesson 22

22-malloc/cpu/ports.c

@@ -0,0 +1,37 @@
+#include "ports.h"
+
+/**
+ * Read a byte from the specified port
+ */
+u8 port_byte_in (u16 port) {
+    u8 result;
+    /* Inline assembler syntax
+     * !! Notice how the source and destination registers are switched from NASM !!
+     *
+     * '"=a" (result)'; set '=' the C variable '(result)' to the value of register e'a'x
+     * '"d" (port)': map the C variable '(port)' into e'd'x register
+     *
+     * Inputs and outputs are separated by colons
+     */
+    __asm__("in %%dx, %%al" : "=a" (result) : "d" (port));
+    return result;
+}
+
+void port_byte_out (u16 port, u8 data) {
+    /* Notice how here both registers are mapped to C variables and
+     * nothing is returned, thus, no equals '=' in the asm syntax 
+     * However we see a comma since there are two variables in the input area
+     * and none in the 'return' area
+     */
+    __asm__ __volatile__("out %%al, %%dx" : : "a" (data), "d" (port));
+}
+
+u16 port_word_in (u16 port) {
+    u16 result;
+    __asm__("in %%dx, %%ax" : "=a" (result) : "d" (port));
+    return result;
+}
+
+void port_word_out (u16 port, u16 data) {
+    __asm__ __volatile__("out %%ax, %%dx" : : "a" (data), "d" (port));
+}