When setting the number of clocks taken by ALU operations to zero and cycle accurate bus cycles we appear to get a nearly 1.5X speed improvement over the stock 8088. This leaves no gaps between 8088 bus cycles which would normally occur while the ALU operations are completing. This may be the theoretical maximum speed attainable when all instructions and data use motherboard resources that use the 8088’s bus interface. When we try using internal memory for RAM and ROM which are not sensitive to the 4.77Mhz clock this speed boost should increase dramatically.
My latest project, the The MCL86+, is a CPU replacement board which uses a Teensy 4.1 microcontroller board to emulate the Intel 8088 microprocessor which is used in the original IBM Personal Computer models 5150 and 5160.
The MCL86+ hardware supports only the Maximum mode at this time which will work for the IBM Personal Computers and most of the XT clones and can run cycle accurate at 4.77Mhz.
The 8086 emulation is written in simple C code and can be configured to run in clock accurate as well as accelerated modes. There is plenty of RAM and ROM to support multiple BIOS images, internal RAM, and optional peripherals. The MCL86’s 8086 Execution unit (EU) is abstracted from the Bus Interface Unit (BIU) which makes supporting different bus interfaces easy. In fact, the code was debugged using a command-line stub for the BIU so that I could run it on my laptop!
I will post all of the source code to GitHub soon. I recently was able to boot to DOS and I am currently working on cleaning up the code, zeroing in on the cycle accuracy, and testing different software applications on various motherboards.
The latest news is that it can boot multiple versions of DOS, run various CPU speed tests, and I have played Archon, Flight Simulator 2, JumpMan, and a few other tools with no problems (yet).
More information coming soon…
This is the picture of the board which fits nicely in an IBM 5160, hovering over the 8087 socket.
I used KiCad for the schematic entry and PCB layout. Here is the 3D view:
MicroCore Labs 