Syncing the clocks

Syncing the clocks
Few months ago I made what I believe was the last hardware fix in the current build. I realized only yesterday that I have not posted the revised schematics, so today’s post is to make up for it. The fix itself is a simple but important update to the clock generation circuit. In my post from ...

Wait states 1

Wait states
My recent order of boards from BatchPCB got lost somewhere on its way. I am not sure if it was lost by USPS, at customs or at my local post office here in Warsaw, but the fact is I am still waiting for a new memory and UARTs boards. BatchPCB is re-doing my order now ...

More bugs

More bugs
Recent exercises with UART revealed two interesting bugs in my design. Both problems were resolved and I am documenting them here for dairy keeping. Implicit memory reads It is something I have discovered only now, playing with real devices attached to the CPU’s expansion bus. The problem is related to side effects of implicitly reading ...

Race conditions 2

Race conditions
I think I am running into all kinds of logic design pitfalls possible. Although it slows me down quite a bit, I consider this an excellent educational experience. Race conditions is what hit me this time. In general a race condition is a state of a logic system in which particular logic gate temporarily outputs ...

Faults and interrupts

Faults and interrupts
It’s been a while since I last posted and in fact I haven’t been working on the project much recently. My job is keeping me extremely busy again. Still, I made some progress on faults and interrupts mechanism for the CPU which I never managed to document. I simulated and tested the design, so it is time to ...

Random findings and tweaks from testing

While I am nearing completion of a CPU test suite, there are some findings I would like to share here, as they changed some of my previous assumptions. Besides, they are important aspects for future programming, so I need to remember these. Carry/borrow flag. First thing is how the ‘181 ALU handles the carry flag ...