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Old 12-31-2008, 11:46 PM
Pops Racer Pops Racer is offline
Junior Member
Join Date: Nov 2007
Location: Lakelands, NSW, Australia
Posts: 15
Default Basic Altair Kit completed

Hi all,

Finished my Altair 8800 yesterday! Hurray! It would not run when first powered up, examine did not work, reset caused machine to free run. So I suspected there was a problem with D/C to Motherboard wiring.

Turned the unit over and there was the culprit! Solder bridge between two motherboard pins!

All good now. Got the 1024K board in and memory tested.

Here's some fun behavior for the 8080. I use the HLT instruction in simulation to stop the sim and let me know that the code is finished. Ha! Doing it on a real 8080 causes the cpu to enter an 'undefined' state. Stumped me for a while.

From the manual:- "Operation: Implementation of the HLT instruction steps the PC to the next instruction address and stops the computer until an interrupt occurs. The HLT instruction should not normally be implemented when a DI instruction has been executed. Since the DI instruction causes the computer to ignore interrupts, the computer will not operate again until the main power switch is turned off and then back on."

The default power up state is interrupts disabled. (INTE LED is OFF), so the HLT really freezes the computer - But in the Arrowhead Tips section (page 5) it says, "To reset the computer you have to hold the stop switch raised while raising the reset switch. Release the reset switch first. (No, we don't know why, but it's traditional!)"

I guess it is a quirk of the D/C design and the 8080 CPU (Looks like assert PRDY and toggle PRESET causes the CPU to resume normal operation?)

Anyways - heaps of fun just mucking around with some of the basic instructions. Tried reading the sense switches with IN A,377 and then storing the data somewhere. Works really well. Good thing the jumper was added to 'read' the sense switches!

Moving on to the 88-4MCS and 2SIO using only original MITS documentation.

Kudos to Grant for this amazing kit.

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