Cepums-86 Emulator project

Cepums-86 is an x86 emulator built to help design a homebrew clone of the IBM PC XT microcomputer

Emulated hardware

• Intel 8086 microprocessor
• Intel 8042 PS/2 keyboard controller (no mouse)
• Intel 8254 programmable interval timer
• MDA graphics adapter
• Real-time clock and CMOS
• Floppy disk controller with support for 4 drives (heavy WIP)

Current state

• Builds on Windows and Linux
• Has enough implemented instructions to run the 8088_bios BIOS (modified fork) Instructions are implemented as they're hit
• Flags may not be set correctly by all instructions
• Instructions themselves should be mostly correct but some mistakes may have slipped in
• There is a macro called STRICT8086INSTRUCTIONSET to make the emulator invalidate instructions not present in the original Intel 8086/8088 documentation. Wikipedia lists some instructions (e.g. 0x83/1 OR variant) as "since 80186" in the 8086/8088 category. I don't have physical hardware to test these on, but NASM appears to use these instructions when set to 8086 mode so I'm unsure if these exist in real hardware
• While the clocks should run at their correct frequencies (4.77 MHz for CPU, 1.19 MHz for PIT), instruction timings aren't implemented so instructions execute faster than on a real processor
• Floppy disk controller isn't finished so it can't boot yet
• Port 80h is used by BIOS to output debug information
• Many parts of the system (like the PIC and speaker) are just stubs and don't have any functionality
• Interrupts are supported but are kinda clunky to use
• Keyboard activity is relayed to the emulator but certain keys might cause crashes
• CMOS settings aren't kept between reboots - they are hardcoded and should pass the checksum check

Why?

Because I've wanted to build something like this for a while. And, this should give me enough information to help design my own microcomputer

How do I build this?

Installation should be relatively easy. First of all, you need the BIOS which is located here. There's a Makefile there that assembles it. If you're on a platform without make, you can use nasm -DMACHINE_DUCK -O9 -f bin -o bios.bin -l bios.lst bios.asm to build it.

Then you have to build the emulator itself. The build system can be generated on Windows by running the included batch script Generate-Win.bat. The batch script should be modified with the Visual Studio version that you have On Linux, you'll need to install premake5 and run premake5 gmake2 to get a Makefile. Then you can build it either with Visual Studio or by running make (depending on your platform). Then you'll need to copy over the bios.bin file and get yourself an IBM VGA font and rename it to default-font.bin.

Additionally on Windows, you'll need to download SDL2 and put SDL2.dll in the same directory.