implement serial port output

Makefile

@@ -97,7 +97,7 @@ $(OBJ_DIR)/%.o: $(SRC_DIR)/%.s
 	$(AS) $(ASFLAGS) -o $@ $<
 	$(OBJCOPY) --remove-section .note.gnu.property $@
 
-MBR_TEST_OBJ		=	obj/mbr_test.o obj/print.o
+MBR_TEST_OBJ		=	obj/mbr_test.o obj/serial.o
 
 mbr_test: $(MBR_TEST_OBJ) $(DISK)
 	$(LD) -T src/mbr_test.ld -o $(BIN_DIR)/mbr_test.bin $(MBR_TEST_OBJ)

src/mbr_test.s

@@ -30,9 +30,10 @@
     xor %sp, %sp
 
 .entry:
-    push $msg
-    call print
+    call serial_init
 
+    mov $msg, %si
+    call prints
 .halt:
     jmp .halt
 

src/serial.s

@@ -0,0 +1,79 @@
+.section .text
+.code16
+
+.globl serial_init
+.globl serial_putc
+.globl prints
+
+.equ    CONFIG_DIVISOR,     1  # we want maximum baud rate
+.equ    CONFIG_DATA_BITS,   8
+.equ    CONFIG_STOP_BITS,   1
+.equ    CONFIG_PORT,        0x3F8
+
+.equ    SERIAL_R_BUFFER,    CONFIG_PORT + 0
+.equ    SERIAL_W_BUFFER,    CONFIG_PORT + 0
+.equ    SERIAL_RW_IER,      CONFIG_PORT + 1
+.equ    SERIAL_DLAB_L,      CONFIG_PORT + 0
+.equ    SERIAL_DLAB_H,      CONFIG_PORT + 0
+.equ    SERIAL_R_II,        CONFIG_PORT + 2
+.equ    SERIAL_W_FIFO,      CONFIG_PORT + 2
+.equ    SERIAL_RW_LCR,      CONFIG_PORT + 3
+.equ    SERIAL_RW_MCR,      CONFIG_PORT + 4
+.equ    SERIAL_R_LSR,       CONFIG_PORT + 5
+.equ    SERIAL_MSR,         CONFIG_PORT + 6
+.equ    SERIAL_RW_SCRATCH,  CONFIG_PORT + 7
+
+# arg 1 - serial port base
+serial_init:
+    # enable DLAB to setup baud divisor
+    mov $(1 << 7), %al
+    mov $SERIAL_RW_LCR, %dx
+    out %al, %dx
+
+    # divisor low byte
+    mov $(CONFIG_DIVISOR & 0xFF), %al
+    mov $SERIAL_DLAB_L, %dx
+    out %al, %dx
+
+    # divisor high byte
+    mov $(CONFIG_DIVISOR >> 8), %al
+    mov $SERIAL_DLAB_H, %dx
+    out %al, %dx
+
+    # config port, but also keep high bit zero to clear DLAB
+    mov $((CONFIG_STOP_BITS << 2) | (CONFIG_DATA_BITS & 0b11)), %al
+    mov $SERIAL_RW_LCR, %dx
+    out %al, %dx
+
+    ret
+
+# al - data byte
+# dx overwritten
+serial_putc:
+    # write byte
+    mov $SERIAL_W_BUFFER, %dx
+    out %al, %dx
+
+    # poll state
+    mov $SERIAL_R_LSR, %dx
+.poll:
+    in %dx, %al
+    and $(1 << 6), %al
+    jz .poll
+
+    ret
+
+# ds:si - asciz
+prints:
+    push %dx
+    cld
+.putc:
+    lodsb
+    or %al, %al
+    jz .end
+
+    call serial_putc
+    jmp .putc
+.end:
+    pop %dx
+    ret