Mippy

Summary

Mippy is a Forth CPU that I built with TTL chips.

Goals of Mippy

Mippy was my first electronics project. With this project I wanted to learn more about processor architecture, design, and implementation. The design of Mippy is open to others. Ths web site will have all the schematics of Mippy. Also, I will be sure to state when I made bad decisions and mistakes. This is, after all, a learning project. ;-)

Architectural Summary

Mippy is a 1MHz, 16 bit Forth machine built from scratch using 74HCT00 series TTL chips. The data bus and address bus are separate, each 16 bits wide. Mippy supports:

• two stacks: data stack & return stack
• a full ALU
• branches
• instructions with embedded immediate values
• interrupts
• micro-code sequencer

There are only 3 programmer visible registers and one state bit:

• 16 bit Program Counter (PC)
• 16 bit data Stack Pointer (SP)
• 16 bit Return stack Pointer (RP)
• Interrupts Enabled bit

Instructions are decoded into a micro-code address and fed into the sequencer.

Brief History

Mippy was born when I had some time after leaving a job. I had a bunch of free time on my hands and decided, like everybody else in the computer world, that I wanted to build a CPU to my specifications. I read the PDP-11 specifications (only later did I learn about the whole PDP family and its architectural beauty). Thinking about what to do with a CPU if I built one, I thought about the frustrations of a development environment. Ideally, the system would be self hosted. This requires I/O (a terminal would suffice for my needs), some sort of storage (a serial line to a host system would suffice. But, the software: Assembler? That's a tough environment to do much work in. I know, I know: lots have done it before me. Sure, and I've written my share of assembler (6502, 68000, PowerPC, Alpha, and MIPS64). But, that wasn't what I'd want. C wasn't ideal for two reasons: first, it required an assembler (because of low-level code that can't be expressed in high level C, and really a C compiler should generate assembly for review) and a C compiler is a bear (now there's the Icc, but at the time I hadn't seen it). Basic is of course out as a systems language because it depends on the primitives that it would need to implement. Then I thought about Forth. Hmmm. There's a system that's fully self-contained. The assembler, compiler, and editor are all part of the same environment. Further, a Forth system can usually be developed in very few K of memory. Indeed, a Forth language on top of whatever processor architecture I devised would be my goal. From there I could build compilers (C and Pascal leap to mind). That's when it occured to me, why not just build a Forth CPU?

Mippy's Name

Last, you might be wanting to ask, "Where did the name Mippy come from?" Early in the development phase I used a Basic Stamp II as a front side bus. Each CPU clock cycle was initiated by the Basic Stamp. After strobing the clock, the stamp would read the Data Bus, Address Bus, and control signals and send them to a Linux host system. The software on the Linux system then decided what course of action to take based on the bus state. It then shipped back a response over the serial port and that was then fed into the front side bus and another CPU clock cycle was initiated. That's a terribly slow process. Roughly one clock cycle per second. A friend pointed out that there are only 32 million seconds in a year, so my computer couldn't be that fast (by comparison modern processors are operating at 1, 2, and 3 billion clock cycles per second). That's when I decided that my CPU was capable of Millions of Instructions Per Year: MIPY. Spelling it Mippy just seemed better.

Demonstrations

Here is the text from the demo software that dad and I showed at the 5th Annual Vintage Computer Fair.

: dia' ( width -- )
	rtz
	dup 1 = over 2 = or
	if 'C' else '!' endif
	swap 1 - dia'
;
: dia-emit ( SENTINEL char char char ... char -- )
	rtz emit dia-emit
;

: dia ( width -- )
	SENTINEL swap dia'
	dia-emit
;

And here's the output of that algorithm (actually this isn't true):

OK 9 diamond
     C
    CC!
   CC!!!
  CC!!!!!
 CC!!!!!!!
CC!!!!!!!!!
 CC!!!!!!!
  CC!!!!!
   CC!!!
    CC!
     C
OK