ao486 - x86 Compatible Verilog Core

The ao486 is an x86 compatible Verilog core implementing all features of a 486 SX.
The core was modeled and tested based on the Bochs software x86 implementation.
Together with the 486 core, the ao486 project also contains a SoC capable of
booting the Linux kernel version 3.13 and Microsoft Windows 95.

Current status

 

• 31 March 2014 - initial version 1.0.
• 19 August 2014 - driver_sd update, ps2 fix.

Links

 

• ao486 project on github.com: http://github.com/alfikpl/ao486.

Features

The ao486 processor model has the following features:

• pipeline architecture with 4 main stages: decode, read, execute and write,
• all 486 instructions are implemented, together with CPUID,
• 16 kB instruction cache,
• 16 kB write-back data cache,
• TLB for 32 entries,
• Altera Avalon interfaces for memory and io access.

• ao486 processor,
• IDE hard drive that redirects to a HDL SD card driver,
• floppy controller that also redirects to the SD card driver,
• 8259 PIC,
• 8237 DMA,
• Sound Blaster 2.0 with DSP and OPL2 (FM synthesis not fully working). Sound output redirected to a WM8731 audio codec,
• 8254 PIT,
• 8042 keyboard and mouse controller,
• RTC,
• standard VGA.

All components are modeled as Altera Qsys components. Altera Qsys connects all parts together, and supplies the SDRAM controller.

The ao486 project is currently only running on the Terasic DE2-115 board.

Resource usage

The project is synthesised for the Altera Cyclone IV E EP4CE115F29C7 device. Resource utilization is as follows:

Unit	Logic cells	M9K memory blocks
ao486 processor	36517	47
floppy	1514	2
hdd	2071	17
nios2	1056	3
onchip for nios2	0	32
pc_dma	848	0
pic	388	0
pit	667	0
ps2	742	2
rtc	783	1
sound	37131	29
vga	2534	260

,

The fitter raport after compiling all components of the ao486 project is as follows:

 

Fitter Status : Successful - Sun Mar 30 21:00:13 2014<br /> Quartus II 64-Bit Version : 13.1.0 Build 162 10/23/2013 SJ Web Edition<br /> Revision Name : soc<br /> Top-level Entity Name : soc<br /> Family : Cyclone IV E<br /> Device : EP4CE115F29C7<br /> Timing Models : Final<br /> Total logic elements : 91,256 / 114,480 ( 80 % )<br /> Total combinational functions : 86,811 / 114,480 ( 76 % )<br /> Dedicated logic registers : 26,746 / 114,480 ( 23 % )<br /> Total registers : 26865<br /> Total pins : 108 / 529 ( 20 % )<br /> Total virtual pins : 0<br /> Total memory bits : 2,993,408 / 3,981,312 ( 75 % )<br /> Embedded Multiplier 9-bit elements : 44 / 532 ( 8 % )<br /> Total PLLs : 1 / 4 ( 25 % ) ,

 

The maximum frequency is 39 MHz. The project uses a 30 MHz clock.

CPU benchmarks

The package DosTests.zip from http://www.roylongbottom.org.uk/dhrystone%20results.htm was used to benchmark the ao486.

Test	Result
Dhryston 1 Benchmark Non-Optimised	1.00 VAX MIPS
Dhryston 1 Benchmark Optimised	4.58 VAX MIPS
Dhryston 2 Benchmark Non-Optimised	1.01 VAX MIPS
Dhryston 2 Benchmark Optimised	3.84 VAX MIPS

,

Running software

The ao486 successfuly runs the following software:

• Microsoft MS-DOS version 6.22,
• Microsoft Windows for Workgroups 3.11,
• Microsoft Windows 95,
• Linux 3.13.1.

BIOS

 

The ao486 project uses the BIOS from the Bochs project (http://bochs.sourceforge.net, version 2.6.2). Some minor changes were required to support the hard drive.

The VGA BIOS is from the VGABIOS project (http://www.nongnu.org/vgabios, version 0.7a). No changes were required. The VGA model does not have VBE extensions, so the extensions were disabled.

NIOS2 controller

 

The ao486 SoC uses a Altera NIOS2 processor for managing all components and displaying the contents of the On Screen Display.

The OSD allows the user to insert and remove floppy disks.

License

All files in the following directories:

• rtl,
• ao486_tool,
• sim.

• bochs486,
• bochsDevs.

The binary file sd/fd_1_44m/fdboot.img is taken from the FreeDOS project.

The binary file sd/bios/bochs_legacy is a compiled BIOS from the Bochs project.

The binary file sd/vgabios/vgabios-lgpl is a compiled VGA BIOS from the vgabios project.