io ports

← Home

I/O Ports

The hardware port numbers MyOS-Simple reads and writes — keyboard, CMOS — plus the real-mode BIOS interrupts it leaves behind.

The x86 has a second address space, separate from memory, reached with the in and out instructions. MyOS-Simple uses exactly four ports: two for the 8042 keyboard controller and two for the CMOS real-time clock. Everything else it talks to — the display — is memory-mapped, not port-mapped.

Port map

Port	Direction	Meaning	Used by
0x60	R / W	Keyboard data: read a scancode, or write a command/byte to the device	ps2-keyboard
0x64	R = status, W = command	Read the 8042 status register; write a controller command	ps2-keyboard
0x70	W	CMOS register select (bit 7 also controls NMI disable)	rtc
0x71	R / W	CMOS data: read/write the register selected via 0x70	rtc

Defined as KEYBOARD_DATA_PORT 0x60 / KEYBOARD_STATUS_PORT 0x64 (keyboard.h:15-16) and CMOS_ADDRESS 0x70 / CMOS_DATA 0x71 (rtc.h:14-15).

8042 status register bits (read from 0x64)

Bit	Mask	Name	Meaning when set
0	0x01	Output buffer full	A byte (scancode) is waiting in 0x60 — safe to read
1	0x02	Input buffer full	The controller has not consumed your last write to 0x60/0x64 — wait before writing

KEYBOARD_OUTPUT_FULL 0x01 and KEYBOARD_INPUT_FULL 0x02 in keyboard.h:19-20. Polling waits for bit 0 before reading a scancode — this is polling, not interrupts.

The CMOS register file (reached through 0x70/0x71)

You select a register by writing its index to 0x70, then read or write the value through 0x71. The RTC fields MyOS-Simple uses (rtc.h:18-31):

Register	Index	Field
RTC_SECONDS	0x00	Seconds
RTC_MINUTES	0x02	Minutes
RTC_HOURS	0x04	Hours
RTC_WEEKDAY	0x06	Day of week
RTC_DAY	0x07	Day of month
RTC_MONTH	0x08	Month
RTC_YEAR	0x09	Year
RTC_CENTURY	0x32	Century (if present)
RTC_STATUS_A	0x0A	Status A (contains the UIP flag)
RTC_STATUS_B	0x0B	Status B (24-hour / binary-vs-BCD flags)

Status B flags: RTC_24HOUR 0x02, RTC_BINARY 0x04.

VGA framebuffer is memory-mapped, not a port

The display is not reached with in/out. The VGA text framebuffer lives at the physical address 0xB8000 (MMIO); the kernel writes characters and attribute bytes directly into that memory. See the memory map and VGA text mode.

Real-mode BIOS interrupts (not ports)

Before the switch to protected mode, the bootloader uses BIOS services via software interrupts. These are only available in real mode — once CR0.PE is set they are gone, which is exactly why the C stages talk to ports directly.

Interrupt	Service	Used for
int 0x10	Video	Clear screen, teletype print in the bootloader
int 0x13	Disk	Read the kernel sectors (CHS) from 0x1000
int 0x16	Keyboard	Stage 1 keyboard input

Accessing ports from C

The C stages wrap in/out in inline-assembly helpers (rtc.c:17-25):

static inline unsigned char inb(unsigned short port) {
    unsigned char result;
    __asm__ __volatile__("inb %1, %0" : "=a"(result) : "Nd"(port));
    return result;
}

static inline void outb(unsigned short port, unsigned char data) {
    __asm__ __volatile__("outb %0, %1" : : "a"(data), "Nd"(port));
}

Constraint	Role
"=a"(result)	Output: the read byte comes back in AL
"a"(data)	Input: the byte to write goes in AL
"Nd"(port)	Port number: an 8-bit immediate (N) or DX (d)

💡 Tidbit: The "Nd" constraint exists because the in/out opcodes can take the port either as an 8-bit immediate (ports 0–255) or in the DX register. Ports 0x60–0x71 fit in a byte, so the assembler can encode them inline; larger port numbers would force the DX form.

⚠️ Caveat: Reading 0x60 without first checking status bit 0 of 0x64 returns stale or garbage data. The polling loop must spin on output-buffer-full before consuming a scancode, or keystrokes will be dropped and duplicated.

💡 Tidbit: Bit 7 of the value written to 0x70 is the NMI-disable bit. Writing a bare register index (0x00–0x7F) leaves NMIs enabled; some firmware code is careful to OR in 0x80 while touching CMOS. MyOS-Simple writes plain indices.

See also

• PS/2 keyboard & the 8042 — ports 0x60/0x64
• Scancodes and scancode tables
• CMOS / RTC — ports 0x70/0x71
• BIOS services — the real-mode interrupts
• Memory map — the MMIO framebuffer at 0xB8000
• Glossary
• Home