RAM and Decoder

A project log for Homebrew Machine 2

An 8 bit breadboard computer based on the Malvino SAP-2.

Brandon ReinhartBrandon Reinhart 11/25/2019 at 17:391 Comment

Been a while since an update. Vacation took me away from the home for a bit, but I have been working on finishing a few modules.

For RAM, I decided to create a control line that would select between RAM being setup by the address register or the program counter. This provides for addressing modes that want to lookup a 16 bit address at PC+1 and PC+2. I could have used some kind of internal register for this, but the RAM mux seemed like a more direct approach and I think covers all of my instruction architecture needs.

I accidentally ordered 512kx8 OTP EPROMs instead of the RAM chips I wanted, so right now I'm only running an 8k RAM. I have enough address lines for a 64k RAM, which should arrive this week. I couldn't actually find a 64k RAM chip on digikey, so I ordered 128k RAMS. Maybe I can add a bank line later.

The RAM tests out and reads and writes, address lines work correctly:

I ended up placing the PC above the RAM and the AD below. The 8K RAM label is on top of the RAM chip and to the left of it are 4 quad mux chips which select between PC and AD.

Once that was done I added the instruction register (IR) and a line of EEPROMs that will form the instruction decode logic. Instructions will be decoded by combinatorial lookup instead of microcode. Each EEPROM has 8 output lines giving me 32 control lines (for now). 

I created a homemade EEPROM burner which I'll use to program each EEPROM.

Instruction bits are [ 8 bits ] [ 4 bits ] where the 8 bits are the OP code and the 4 bits are time step, so these uniquely address 256 opcodes with 16 time steps. I won't need 16 time steps, so it's possible I could use a bit on something else.

In the image above, the EEPROMs are setup to always output and never write. There's a little 4 bit counter that drives the time step.

And here is the overall computer so far:

The tasks remaining:

Then write some programs to test the range of functionality.

I'll need to add a flag status register, but I figured I'd wait until I was creating those instructions. Flags should be able to bolt onto the existing design without much hassle.

If I get through that, I'll move onto the video card. Long term project path is to create a video card and an interrupt based keyboard interface.


Brandon Reinhart wrote 11/25/2019 at 17:43 point

An alternative to boot loading from an EEPROM is to boot load from an arduino that transfers the code/kernel to the RAM. Maybe if I get a kernel I like, later, I can use those OTP ROMs and just have a kernel ROM bank!

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