Intel 8080 IO Ports

The Intel 8080 CPU can address up to 256 input ports and 256 output ports; allowing for virtually unlimited system expansion. Access to the ports is via the IN and OUT Intel 8080 CPU instructions, see page 38.

Intel 8080 IO ports and peripherals

Altair emulator software-enabled ports

The Altair emulator uses Intel 8080 IO ports to provide time services, random numbers, system information, file transfer, persistent environment variables, weather data, and an OpenAI-compatible chat client.

• You can access Intel 8080 IO ports from BASIC, C, and Assembly programming languages, and directly using Intel 8080 opcodes. See Using Intel 8080 Input Output ports.
• I/O is routed by port_drivers/io_ports.c to per-feature drivers in the same folder (time_io.c, utility_io.c, weather_io.c, files_io.c, environment_io.c, chat_io.c).
• You can extend the Altair 8800 by adding additional IO port functions — drop a new *_io.c driver into port_drivers/ and route the port range from io_ports.c.

Altair Emulator IO Port SDK

Sample code in the Apps/SDK folder demonstrates how to call these IO ports from BDS C. See for example DXENV.C (environment variables) and DXTERM.C (terminal helpers). These examples are also useful references for LLM-assisted development.

Response read port

Many ports return a multi-byte string. The convention is:

1. The CP/M program writes a request byte (or selector) to the output port for the feature.
2. The driver fills an internal response buffer and the program reads bytes from port 200 until a NUL (0x00) is returned.

Port	Direction	Description
200	IN	Read next byte of the most recently loaded response. NUL terminates.

Timer ports (time_io)

Port	Dir	Port data	Loads
24	OUT	0-255	Set timer 0 period high byte (milliseconds)
25	OUT	0-255	Set timer 0 period low byte (milliseconds) and start timer
26	OUT	0-255	Set timer 1 period high byte (milliseconds)
27	OUT	0-255	Set timer 1 period low byte (milliseconds) and start timer
28	OUT	0-255	Set timer 2 period high byte (milliseconds)
29	OUT	0-255	Set timer 2 period low byte (milliseconds) and start timer
30	OUT	0-255	Set seconds timer period and start timer
24-29	IN	-	Query corresponding millisecond timer (non-zero while running, 0 when expired)
30	IN	-	Query seconds timer (non-zero while running, 0 when expired)

Stopwatch ports (time_io)

Passive elapsed-time counters with optional second or millisecond resolution. Unlike the countdown timers above, a stopwatch never expires — it simply measures the time since it was started. The elapsed value is returned as an unsigned 32-bit count, emitted big-endian as 4 raw bytes (the BDS C long byte layout: MSB first) and read back via port 200.

Port	Dir	Port data	Loads (read with port 200)
37	OUT	0	Start/reset stopwatch 0 (records current time; no response)
37	OUT	1	Latch stopwatch 0 elapsed seconds as a 4-byte big-endian long
37	OUT	2	Latch stopwatch 0 elapsed milliseconds as a 4-byte big-endian long
38	OUT	0	Start/reset stopwatch 1 (records current time; no response)
38	OUT	1	Latch stopwatch 1 elapsed seconds as a 4-byte big-endian long
38	OUT	2	Latch stopwatch 1 elapsed milliseconds as a 4-byte big-endian long
39	OUT	0	Start/reset stopwatch 2 (records current time; no response)
39	OUT	1	Latch stopwatch 2 elapsed seconds as a 4-byte big-endian long
39	OUT	2	Latch stopwatch 2 elapsed milliseconds as a 4-byte big-endian long

char elapsed[4];        /* BDS C long: elapsed[0] = MSB            */

outp(37, 0);            /* start stopwatch 0                       */
/* ... do some work ... */
outp(37, 1);            /* latch elapsed seconds                   */
elapsed[0] = inp(200);  /* read the 4-byte big-endian long back    */
elapsed[1] = inp(200);
elapsed[2] = inp(200);
elapsed[3] = inp(200);  /* elapsed now holds the seconds as a long */

Wall clock and date ports (time_io)

The wall clock requires Wi-Fi and a successful SNTP sync. Before sync these ports return a +<seconds>s boot-relative fallback.

Port	Dir	Port data	Loads (read with port 200)
31	OUT	0	Local date/time, DATE-style (Sun 05/31/2026 17:13:00)
41	OUT	0	Seconds since boot, e.g. 12345
42	OUT	0	UTC date/time, ISO 8601 (YYYY-MM-DDTHH:MM:SSZ)
43	OUT	0	Local date/time using configured timezone, ISO 8601
44	OUT	0	Local long date, e.g. Thursday, 21 May 2026

Utility ports (utility_io)

Port	Dir	Port data	Loads (read with port 200 unless noted)
45	OUT	0	Random number (2 bytes: low, high) on port 200
48	OUT	0	Wi-Fi DHCP hostname string
48	OUT	1	Wi-Fi IPv4 address string
48	OUT	2	Device ID (MAC address) string
49	OUT	0xA5	Reboot the ESP32. Any other data byte is ignored.
70	OUT	0	Firmware version string, e.g. ESP32-S3 Altair8800 (IDF v6.0.3)

Weather ports (weather_io)

Requires an OpenWeatherMap API key and location set via the ENV app (keys WEATHER_API_KEY, WEATHER_LOCATION, WEATHER_UNITS). A background task refreshes data approximately every 20 minutes.

Port	Dir	Port data	Loads
46	OUT	0	City
46	OUT	1	Current conditions main, e.g. Rain
46	OUT	2	Current conditions description
46	OUT	3	Current temperature
46	OUT	4	Current humidity
46	OUT	5	Current wind
46	OUT	6	Forecast main
46	OUT	7	Forecast description
46	OUT	8	Forecast temperature
46	OUT	9	Forecast valid-at time
46	OUT	10	Age of the last fetch in seconds
47	IN	-	Status: 0 = no data, 1 = fetching, 2 = ready, 3 = error

Selected field is read byte-by-byte from port 200 until a NUL is returned.

Remote file transfer ports (files_io)

The Altair emulator can pull files from a remote FT server (or from the bundled MCP server) over Wi-Fi. The FT CP/M command on drive B: wraps these ports.

Port	Dir	Port data	Loads
60	OUT	1	SET_FILENAME — start a new filename transfer on port 61
60	OUT	3	REQUEST_CHUNK — request next 256-byte data chunk
60	OUT	4	CLOSE — close the current transfer
61	OUT	ASCII	Filename character (NUL-terminated)
60	IN	-	Status: 0 = idle, 1 = data ready, 2 = EOF, 3 = busy, 255 = error
61	IN	-	Chunk byte (or leading count byte, per FT protocol)

Environment variable ports (environment_io)

Persistent key/value storage backed by ESP32 NVS. The ENV CP/M command and the DXENV.C SDK wrapper use these ports.

Port	Dir	Port data	Loads
71	OUT	0	Reset request buffer
71	OUT	1	INIT — open NVS namespace
71	OUT	2	GET — request preceded by key\0 on port 72; reply on port 200
71	OUT	3	SET — request preceded by key\0value\0 on port 72
71	OUT	4	DELETE — request preceded by key\0 on port 72
71	OUT	5	LIST — list all key/value pairs on port 200
71	OUT	6	COUNT — number of entries on port 200
71	OUT	7	CLEAR — delete all entries
71	OUT	8	EXISTS — request preceded by key\0 on port 72
71	OUT	9	EXECUTE — run a CLI command line previously written to port 72
72	OUT	ASCII	Append byte to request buffer
71	IN	-	Last status code: 0 = OK, -1 open, -2 read, -3 write, -4 full, -5 not found

OpenAI-compatible chat ports (chat_io)

Requires CHAT_OPENAI_KEY (and optionally CHAT_OPENAI_URL, CHAT_OPENAI_MODEL, CHAT_SYSTEM_PROMPT) set via the ENV app.

Port	Dir	Port data	Loads
120	OUT	0	Trigger / start a request
121	OUT	ASCII	Append byte to the user request buffer
122	OUT	0	Reset response state for the next streaming read
120	IN	-	Trigger status (acknowledged)
123	IN	-	Stream status: 0 = EOF, 1 = waiting, 2 = data ready
124	IN	-	Next byte of streamed response when status = data ready

Using Intel 8080 Input Output ports

The following code snippets use the Intel 8080 IO ports. The code samples included on the CP/M boot disk expand on these snippets.

Assembler access to Intel 8080 IO Ports

The following assembly code demonstrates the use of the Intel 8080 IO port 30 timer. The code sets a 2-second delay, and then waits for the timer to expire. This is a snippet of the SLEEP.ASM sample included on drive B: of the Altair emulator.

      ORG 0100H   ;CP/M base of TPA (transient program area)
      MVI A,2     ;Move 2 to the accumulator to set a 2 second delay
      OUT 30      ;Start timer
LOOP: IN 30       ;Get delay timer state into the accumulator
      CPI 00H     ;If accumulator equal to 0 then timer has expired
      JZ BACK     ;Jump on zero
      JMP LOOP
BACK: RET

BSD C access to Intel 8080 IO Ports

The following C code demonstrates the use of the Intel 8080 IO port 30 timer. The code sets a 1-second delay, and then waits for the timer to expire. This is a snippet of the HW.C sample included on drive B: of the Altair emulator.

outp(30,1);      /* Enable delay for 1 second */
while(inp(30));  /* Wait for delay to expire */

Front panel: reboot the ESP32 in binary

You can also drive ports directly from the Altair front panel (physical kit or the web virtual console) by toggling raw opcodes into memory. The four bytes below load 0xA5 into the accumulator, write it to port 49, and halt — which causes the ESP32 to restart.

Addr	Hex	Binary	Mnemonic	Meaning
0000	3E	0011 1110	MVI A, data	Load next byte into A
0001	A5	1010 0101	data	The reboot magic byte (0xA5)
0002	D3	1101 0011	OUT port	Output A to next byte's port
0003	31	0011 0001	port	Port 49 (31h) — reboot
0004	76	0111 0110	HLT	Halt (never reached; ESP32 resets)

Front panel sequence:

1. Set the address switches to 0000 0000 0000 0000 and press EXAMINE.
2. For each of the five bytes above, set the data switches to the binary value and press DEPOSIT NEXT (the first byte uses plain DEPOSIT).
3. Set address back to 0000 0000 0000 0000, press EXAMINE, then RUN.

The ESP32 reboots almost immediately after the OUT 31h is executed.

Tip: the same reboot is available without toggling switches by running the ENV app on CP/M drive A: and choosing the R (Reboot) menu option.

BASIC access to Intel 8080 IO Ports

The following BASIC code demonstrates the use of the Intel 8080 IO port 30 timer. The code sets a 1-second delay, and then waits for the timer to expire. This is a snippet of the COUNT.BAS sample included on drive A: of the Altair emulator.

Delay IO ports

900 REM This sleep subroutine sleeps or delays for 1 second
1000 OUT 30, 1
1100 WAIT 30, 1, 1
1200 RETURN

Weather IO ports

The following BASIC code selects the current temperature field on port 46 and then reads the string from port 200 byte-by-byte until a NUL is returned. The OpenWeatherMap API key and location must be configured first using the ENV CP/M app.

500 PORT = 46 : REM Weather field selector port
510 PDATA = 3 : REM 3 = current temperature
520 GOSUB 4800 : REM Loads the field, then reads it as a string
530 PRINT RSTRING$
540 END

4800 REM SUBROUTINE READS STRING DATA FROM PORT UNTIL NULL CHARACTER
4900 OUT PORT, PDATA
5000 RSTRING$ = ""
5100 C=INP(200) : REM Read characters until NULL returned
5200 IF C = 0 THEN RETURN
5300 RSTRING$ = RSTRING$ + CHR$(C)
5400 GOTO 5100

System info IO ports

The following BASIC reads the device's Wi-Fi IP address (port 48 selector 1):

100 OUT 48, 1
110 S$ = ""
120 C = INP(200)
130 IF C = 0 THEN GOTO 200
140 S$ = S$ + CHR$(C)
150 GOTO 120
200 PRINT "IP = "; S$

Reboot the ESP32

CP/M cannot rebuild the host on its own — port 49 with the magic byte 0xA5 (165 decimal) triggers an esp_restart(). Any other data byte is ignored, so accidental writes are safe.

outp(49, 0xA5);   /* Reboot the ESP32 after the current OUT completes. */

10 OUT 49, 165   : REM 0xA5 — reboot the ESP32

The ENV CP/M app exposes this as the R (Reboot) menu option with a y/N confirmation prompt.