8088sbc

8088 Single Board Computer

A minimal single-board computer based on the Intel 8088 processor. That's the same processor used in the original IBM PC, but this is just a simple project and doesn't strive for PC/DOS compatibility.

Specs

• 8088 or compatible CPU in minimal mode (5 MHz with 8088, 8 MHz with 8088-2)
  • Clock oscillator at 3x frequency of CPU clock frequency. For 8 MHz operation, uses a 24 MHz half-can crystal oscillator
• 1 MB static RAM (2x AS6C4008-55PCN)
• 32 KB ROM (1x AT28C256-15)
• FTDI-compatible 5V serial port provided by a 16C550 UART (up to 115200 baud)
• 32 column by 4 row LED display (sixteen PDSP-1881 intelligent displays, display board schematic available here)
• Uses five common 74HC chips for glue logic (74HC4017, 74HC132, 74HC573, 74HC75, 74HC139). Does not require any of the Intel 8200-series support chips.
• 5V power via USB-A connector

Memory map

00000h-7FFFFh - RAM
80000h-FFFFFh - bankable (RAM, ROM, expansion 1, or expansion 2)

I/O map

Only bits 7 and 8 are used for decoding.

000h-07Fh - I/O bank 0
    060h-067h - 16550 UART
080h-0FFh - I/O bank 1
100h-17Fh - I/O bank 2
    100h-107h - 128-character display (16x PDSP-1881 or similar)
180h-1FFh - I/O bank 3

More info on display decoding:

100h-17Fh - Flash RAM
900h-97Fh - UDC Address Register
B00h-B7Fh - UDC RAM
D00h-D7Fh - Control Word Register
F00h-F7Fh - Character RAM

Bank switching and input/output

The modem control lines of the 16550 UART are used as general purpose input and output lines.

~OUT1 - bank select bit 0
~OUT2 - bank select bit 1
~DTR  - general purpose output 0 (LED2)
~RTS  - general purpose output 1 (LED3)
~RI   - general purpose input 0
~DCD  - general purpose input 1
~DSR  - general purpose input 2
~CTS  - general purpose input 3

Bank select bits 0 and 1 control which device is mapped into the upper half of the address space:

00 - expansion 1
01 - expansion 2
10 - RAM
11 - ROM (default at power-up)

These are controlled by bits 2 and 3 of the 16550's Modem Control Register. (64h)

Interrupts

The four general-purpose inputs can be set up as inputs by configuring the 16550's Interrupt Enable Register (61h). However, this won't be very useful because the 8088's ~INTA line is not connected to anything, so when an interrupt is received, there is no device to place the vector number on the data bus.

Pressing the NMI button will generate a non-maskable interrupt (INT 04h). The button is debounced in hardware.

Software

Not much here, just enough to test memory and dump the register state. Compiles with NASM (version 2.14.02 used). Running make in the code/ directory generates as 32KB binary file. Running make flash can be used to burn the ROM image to an AT28C256 EEPROM chip using a MiniPro TL866II+ with the minipro utility.

(The TL866II+ is around $60 USD and widely available, if you don't have one, you should get one!)

1. Power LED will light up.
2. OUT1 LED will blink four times. (Testing 8 segments of lower RAM)
3. OUT0 will light.
4. OUT1 LED will blink four times. (Testing 8 segments of upper RAM)
5. Hello 8088! will be printed to the serial port. (115200 8N1)
6. OUT0 and OUT1 LEDs will toggle with 50% duty cycle. (verify with scope)
7. Pressing the NMI button will print the register state:

AX=F001 CS=F000 IP=82E0      NMI
BX=FFF0 DS=F000 SI=82F5     --I-
CX=0000 ES=0000 DI=0024 --------
DX=F202 SS=7000 SP=FFE6 BP=FFFF

API

Interrupt vector 06h points to the character output routine. If using the PDSP-1881 LED display, this can be changed to disp_outch_int to write to the display instead of the serial port.

INT 07h prints the null-terminated string in CS:SI.

INT 08h is a rudimentary formatted-output routine. See output.inc.

Future work

Probably none. This was just a quick project and I probably won't work on it long-term because... honestly, I'm just not a fan of 8086 assembly language and its weird 16-bit segmented model. Mainly, I had these chips lying around and wanted to do something with them.

About

Open hardware, released under the terms of the 3-clause BSD license.

Copyright 2020 Matt Sarnoff.

http://twitter.com/txsector

http://msarnoff.org