Add ports files and add use ANSI C.

Makefile

@@ -1,11 +1,11 @@
 ASM=nasm
 BIN=bin
-CFLAGS=-ffreestanding -fno-pie -m32
+CFLAGS=-ffreestanding -fno-pie -m32 -ansi
 CC=gcc
 LD=ld
 LDFLAGS=-melf_i386 -Ttext 0x1000 --oformat binary
 
-OBJ=kernel/kernel_entry.o kernel/kernel.o
+OBJ=kernel/kernel_entry.o kernel/kernel.o kernel/ports.o
 
 all: os-image
 
@@ -29,5 +29,5 @@ kernel/kernel.bin: $(OBJ)
 .PHONY: clean
 
 clean:
-	rm -rf boot/*.bin boot/*.o
+	rm -rf boot/*.bin kernel/*.bin
 	rm -rf kernel/*.o boot/*.o drivers/*.o

kernel/ports.c

@@ -0,0 +1,23 @@
+unsigned char port_byte_in(unsigned char port) {
+	unsigned char res;
+	/*
+	 * `"=a" (res)' means: put result of `al' into `res' variable
+	 * `"d" (port)' means: load `port' into `dx'.
+	 */
+	__asm__("in %%dx, %%al" : "=a" (res) : "d" (port));
+	return res;
+}
+
+void port_byte_out(unsigned char port, unsigned char data) {
+	__asm__("out %%al, %%dx" : : "a" (data), "d" (port));
+}
+
+unsigned short port_word_in(unsigned short port) {
+	unsigned short res;
+	__asm__("in %%dx, %%ax" : "=a" (res) : "d" (port));
+	return res;
+}
+
+void port_word_out(unsigned short port, unsigned short data) {
+	__asm__("out %%ax, %%dx" : : "a" (data), "d" (port));
+}

kernel/ports.h

@@ -0,0 +1,18 @@
+#pragma once
+
+/*
+ * return a byte from a port.
+ */
+unsigned char port_byte_in(unsigned char port);
+/*
+ * write a byte of data to a port.
+ */
+void port_byte_out(unsigned char port, unsigned char data);
+/*
+ * return a word of data from a port.
+ */
+unsigned short port_word_in(unsigned short port);
+/*
+ * write a word of data to a port.
+ */
+void port_word_out(unsigned short port, unsigned short data);