A complete whole-system simulation of the ao486 CPU core and PC architecture, hacked together for learning and understanding the inner workings of Intel 80486 processors and classic PC hardware. This simulator includes a fully functional MS-DOS 6.22 system with working hard disk support.

ao486-sim is an educational tool that simulates:

• Intel 80486-compatible CPU core with full x86 instruction set support
• Complete PC architecture including essential peripherals
• Working MS-DOS 6.22 system
• Real-time VGA graphics for text and graphics
• PS/2 keyboard support in simulation
• IDE/ATA hard disk controller with virtual disk support (raw format)
• Bochs BIOS: Full PC BIOS implementation

• Verilator (5.0 or later)
• SDL2 development libraries
• C++17 compatible compiler
• Make

git clone https://github.com/nand2mario/ao486-sim.git
cd ao486-sim/verilator
make sim

The simulator will start and boot into MS-DOS 6.22. You'll see the familiar DOS prompt after the boot sequence completes.

A few notes:

• Keyboard: You can type directly at the DOS prompt. The simulator converts your key presses into PS/2 scancodes and delivers them to the RTL core.
• When you see "Starting MS-DOS...", pressing any key will speed up the boot process, as DOS is waiting for user input at that stage.
• Output: Watch the terminal window for colored status messages from the BIOS and DOS. These are captured by intercepting specific software interrupts and function calls.
• Exit: To quit, simply close the window or press Ctrl+C in the terminal.
• Hard disk: The hard disk image is not automatically saved; you must manually persist any changes by pressing a key (CMD-s on Mac or WIN-s on Windows). The IDE module (src/soc/ide.v) is based on ao486's original hdd.v, which used an SD card for storage. In this simulator, it has been modified to use a disk image file instead.
• 2MB of main memory is available by default. You can increase this by modifying init_cmos() in main.cpp and SIZE_MB in src/sdram_sim.sv. Note that increasing memory will cause himem.sys initialization to take proportionally longer.
• On an M4 MacBook Pro, the simulation runs at about 0.7 FPS, and booting DOS takes roughly 1.5 minutes.
• There is a known Verilator race condition that can cause Internal Error: ../V3TSP.cpp:353 during compilation. If you encounter this, try running make several times. If the issue persists, remove --threads 2 from the Makefile; the simulation will run a bit slower, but should work reliably.

The dos6.vhd is just the raw sectors of the hard disk. The easies way I found to create and manipulate them is qemu-img as part of QEMU. To create a new image,

qemu-img create my.vhd 32m

Then you can mount the image and install DOS or other software packages from floppies,

qemu-system-i386 -drive file=my.vhd,format=raw,index=0,media=disk -m 64 -L . -fda msdos6_22disk1.img -boot a

Read Makefile for how to use the new hard disk image.

The ao486 CPU implements a 4-stage pipeline:

1. Decode: Instruction decoding and operand preparation
2. Read: Reads in required data from memory.
3. Execute: ALU operations, memory address calculation
4. Write: Memory and register write-backs.

Booting DOS

DOS edit.com

Captured messages from Bochs BIOS

Generate detailed waveforms for analysis:

make trace

This creates a waveform file that can be viewed with GTKWave:

gtkwave waveform.fst

• ao486 project: Original CPU implementation
• Bochs project: BIOS and PC architecture reference

ao486-sim: Learn x86 architecture by running a complete PC system in simulation