Lesson 18

17-video-scroll/drivers/ports.c

@@ -1 +0,0 @@
-../../16-video-driver/drivers/ports.c

17-video-scroll/drivers/ports.c

@@ -0,0 +1,35 @@
+/**
+ * Read a byte from the specified port
+ */
+unsigned char port_byte_in (unsigned short port) {
+    unsigned char 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 (unsigned short port, unsigned char 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__("out %%al, %%dx" : : "a" (data), "d" (port));
+}
+
+unsigned short port_word_in (unsigned short port) {
+    unsigned short result;
+    __asm__("in %%dx, %%ax" : "=a" (result) : "d" (port));
+    return result;
+}
+
+void port_word_out (unsigned short port, unsigned short data) {
+    __asm__("out %%ax, %%dx" : : "a" (data), "d" (port));
+}