68008 SRAM Microcomputer – Free Run


yesterday i searched for some of the parts and put the 68008 on the bread board. According to other projects it is possible to do a "free run" by pulling all data lines to low. Though some people say it's the opcode of NOP it actually is some kind of ORI.

I wanted to check some prerequisites of the project:
  1. Does the CPU work?
  2. Can !DTACK (data acknowledge) be held permanently low?
  3. Can !VPA (valid peripheral address) be used for instruction fetch cycle? 
ad 1: Yes the CPU works. It cycles through it's address space and toggles A19 with 2 Hz.
ad 2: Yes, as i could tell the CPU works. :-)
ad 3: For curiosity: Yes. I believe that bus cycles and internal logic are completely separated and !VPA can be used for any bus cycle.

Next interesting question: Can i put videos in my blog? It seems i can, but for a final verification i probably have to publish this page.

Update: They are just converted into poor animated GIFs. I'll have to find something better... ok, uploaded them to youtube and embedded. back to the roots...

Free Run using !DTACK-terminated bus cycles
To the left is the 68008 on my bread board and wired up to us !DTACK to terminate bus cycles. !DTACK is permanently low (active) and the CPU runs as fast as it can: At 8 MHz it does 2 opcode fetches per µsec or 2,000,000 opcode fetches per second. As the whole address space of the 68008 is 1 MB only, it cycles through it's address space 2 times a second. The most significant address bit A19 should blink with 2 Hz. A19 is the leftmost LED in the video and i hope you can verify that it blinks with 2 Hz. Thanks.

An important result is that the 68008 actually works without deactivating !DTACK after each bus cycle. Though in all timing diagrams bus cycles start with !DTACK high it is actually possible to keep it low the whole time.

Free Run using !VPA-terminated bus cycles
In the second video i used !VPA to terminate the bus cycles instead. This mode is intended to access old (really old!) 6800 peripherals but it seems true that you can terminate any bus cycle with !VPA, even an opcode fetch cycle. It's just slower. I was curious how slow actually, if every bus cycle uses !VPA, because the 68008 data sheet say it can be from 11 to 18 clock cycles long.

Actually the M68000 8-/16-/32-Bit Microprocessors User’s Manual Ninth Edition says 10 to 19 cycles, while the M68000 Family Reference Manual – MC68008 Technical Summary says 11 to 18 cycles.

Buggy timing diagram for the 'best case' !VPA-terminated bus cycle
The latter puzzled me, because of course i started with the 68008 documentation, because that's the CPU i'm using, and i was wondering how fast the 68008 could uninterruptedly access the bus using the !VPA mode as 11 cycles is slower than the period of the free running E signal to which !VPA bus cycles are synchronized. But 10 to 19 makes sense (while 11 to 18 makes not) and i found an unnamed cycle in the 68008 manual's 'best case' chart (between the last 'w' cycle and 'S5') and i believe that someone reviewed the charts, found that the 'worst case' chart was only 18 cycles instead of 19 cycles long, demanded a correction and the missing cycle was added ... to the wrong chart. That's how real world works.

In the second video one bus cycle takes 10 clock cycles instead of 4 and therefore A19 should blink with 2 Hz *4 / 10 = 0.8 Hz instead. I think this approximately true.

In my project the !VPA bus cycle is used to access slow peripherals on the K1 bus. But it is also used during interrupt acknowledge, in order to use an auto vectored interrupt. Now the interesting question is: Does the CPU actually perform a !VPA controlled bus cycle here or a dummy cycle, as it ignores the byte read?

!VPA used in interrupt vector read cycle
My guess was, that it actually does a !VPA controlled slow bus cycle if you activate !VPA, making interrupts approximately 10 clock cycles slower. And finally i found this chart on the last (!) page of the M68000 8-/16-/32-Bit Microprocessors User’s Manual Ninth Edition. The last pages are appendix B which is about interfacing 6800 devices and which are pasted into the document as bitmaps only. :-)

The bus interface performs a slow memory cycle in the (dummy) interrupt vector read cycle if !VPA is activated to request an auto vector interrupt.


  1. Hi.

    Are you planning on turning this into a Full microcomputer, with VIDEO,SOUND,RS232,PS/2 keyboard and stuff ??

    1. It's a K1-Bus project, which means, it's modular.
      A K1-Bus 2xSIO board exists, for the beginning i'll work with a terminal emulation on my Mac connected to the board via RS232.
      But currently the project is stalled.
      I don't know when i'll resume work on it. Maybe on public request. B-)