The Video display card mimics an MDA/CGA text and graphics cards on a standard VGA monitor. The emulated graphics card uses a Raspberry Pi (RPi) running custom software as a video generator. The RPi connects through Serial GPIO to a Z80-SIO/2 UART to interface with the 8088 PCXT bus and BIOS.

• RPi GPIO resources
• BCM2835 and errata (https://elinux.org/BCM2835_datasheet_errata)
• CPU 8088-2 data sheets
• IBM 5160 technical reference
• Z80-SIO UART

• Schematics GitHub URL

The RPi software works in conjunction with the PC/XT BIOS code (https://github.com/eyalabraham/new-xt-bios) The main component in the RPi software is the Frame Buffer. Two options to use the frame buffer, one from Linux and the other on bare-metal RPi with no OS.

Main resource (http://raspberrycompote.blogspot.com/). This module is written to run under a strip-down RPi Linux distribution. The goal is to create a headless driver that is as OS independent as possible so that porting to bare-meta will be as direct as possible. RPi OS installation is tuned to eliminate IO to SD card so that power on/off will not depend on or affect SD card data storage/caching. Possible setup will use TinyCore or Make the root partition read only. This Ada Fruit read only setup seemed best because the read-only mode is select-able with a jumper on the GPIO lines.

(1) This is a special mode for mon88, text 160x64

PC/XT will translate INT 10h calls from the application into a set of bytes sent to the RPi. These bytes will form the control primitives that manage the display. The bytes will be sent as "packets" through Z80-SIO UART channel B that is connected to the RPi UART. Each packet will be delimited by escape characters similar to the Serial IP protocol SLIP, with END escape codes at the start and end of the command primitive packet. This is a simple and efficient way to frame these command packets.

The control packets include:

(1) Character is written to cursor position
(2) Attribute: Attribute byte will be decoded per video mode
(3) XOR-ed with current pixel if bit.7=1
(4) Act on active page
(5) PC/XT can not send partial commands, any trailing bytes should be padded with '0'
(6) Return data format: two bytes {character}{attribute}
(7) same at command #4, but use existing attribute
(8) Return data format: one byte {color_code}
(9) Return data format: six bytes {6}{5}{4}{3}{2}{1}
(10) Two high order bits are command queue: '00' VGA emulation, '01' tbd, '10' tbd, '11' system
(11) A value of 2000h turns cursor off

Functions not listed below will be handled by BIOS, and not transferred to the display controller. The display controller will keep cursor position but will not track movement through writes; cursor reposition will be done by BIOS with command #2. Put Character commands #4 and #6 provide direct character placement parameters, these do not change display controller's stored cursor position. Only command #2 can change display controller's stored cursor position. Therefore, INT 10h functions 0Eh and 13h will output the characters with commands #4 and #6 and then provide one cursor reposition command #2. For visual effect, cursor 'off' and 'on' with command #3 can be used. Command #11 can be used with scroll commands if the text screen needs to be cleared.

• vga.c main module and emulator control loop
• fb.c frame buffer and graphics emulation
• uart.c UART IO driver
• iv8x16u.h 8x16 font bitmap definition bitmap font source for code page 437 characters
• ic8x8u.h 8x8 font bitmap definition bitmap font source for code page 437 characters
• im9x14u.h 9x14 font bitmap definition bitmap font source for code page 437 characters
• config.h compile time module configuration
• util.c utility and helper functions (debug print etc)

An PRi bare-metal implementation of this project is available in my RPi bare-metal code repository