I finally bought a ceramic 8086-1 (from ebay, from China - I'm not sure that it is not fake chip) and 8284 (it make the necessary synchronization signal for the processor from the crystal).

Now I want to check if it works - just run through part of the address space on the LEDs. I don’t know what frequency of the crystal to set so that a person can see how the addresses change and whether the processor will work at such a low frequency? As an option, use circuit in which one processor cycle is performed at the click of a button.

Does anyone have such circuit? I saw such for the Z-80.

P.S. I use this circuit for generating clock signal - 8086

  • 1
    or you can use 80C88, it's a CMOS variant from HMOS type. You can use push button for your clock – Nyoman Karna Dec 24 '20 at 13:09

The HMOS 8086 (as opposed to the CMOS 80C86) has a specified maxmial clock period of 500ns which translates to 2MHz in its datasheet on page 15 . It will fail to work if it is clocked significantly slower. So manual clocking is not possible on that chip.

But what you can do is to generate an arbitrary amount of wait states. My recommendation would be to set up the 8086 together with the 8284 clock generator, because the clock generator generates a properly synchronized ready signal. Then you "just" need to manually apply ready pulses to the 8284 to single-step the system. You should still make sure to meet the RDY timing requirements (check rise times and duration) of the 8284, that's why I put scare quotes around "just".

  • Or another option for check cpu - you can connect the 16/8 dip switch as a ram, and manually set the command to output the value to the port, where the port is a latch of 74hc. Right? – Alex Aug 22 '19 at 7:43
  • Your idea needs the 8086 in minimum mode, a tristate driver (16 bit, so two chips) gated by /RD to provide the fake RAM, a latch gated by /WR to latch data written. You could use the instruction sequence "SAHF / OUT DX,AL" which just takes two bytes, and outputs 02h. We don't know DX after reset, so the value 02 may be output to either the low or the high bus half. If you latch AD0-AD3 and AD8-AD11, one of the nibbles should receive the pattern "0 0 1 0". – Michael Karcher Aug 22 '19 at 8:54

its common to have minimal circuit in the CPU (complete) datasheet

This is first I found:

Here image taken from page 59:

image from pdf above

Looks like this page handles it in more depth:


8086/88 has a TEST input pin (23).

Test input is examined by the ‘‘Wait’’ instruction. If the TEST input is LOW execution continues, otherwise the processor waits in an ‘‘Idle’’ state. This input is synchronized internally during each clock cycle on the leading edge of CLK.

So, you can hardwire the WAIT instruction (0x9B) and generate low pulses on the TEST pin to step the processor.

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