diff --git a/NOTES.md b/NOTES.md index 39045c7..ecf1692 100644 --- a/NOTES.md +++ b/NOTES.md @@ -1,19 +1,24 @@ # NOTES -RAM8 -: trick is to figure out how to mux twice based on address, once for setting the load bit and once for picking the correct output bus. +## RAM8 -RAM64 -: Interesting part here is that the MSB|LSB decision is arbitary. You could use the LSB to pick the RAM8 module, and use the MSB as the address within the RAM8, and it _would still work_. The caller for RAM64 doesn't care how you use the "complete address". It just cares that you return the same thing for that address. +trick is to figure out how to mux twice based on address, once for setting the load bit and once for picking the correct output bus. -RAM512 -: I used the premise of the previous note and decided to index the RAM64 moduls by the LSB instead of the customary MSB. It still works :metal: +## RAM64 -PC -: The Program Counter was tricky. I ended up using a Or8Way (instead of a `Or3Way`, which we don't have) along with `Mux8Way16` to pick the input for the register. Since the Mux has duplicate inputs at this point (supports 8, but we only have 3 special, and 1 neutral case) - this can be optimized by switching to a `Mux4Way16`, along with some other changes. +Interesting part here is that the `MSB|LSB` decision is arbitary. You could use the LSB to pick the RAM8 module, and use the MSB as the address within the RAM8, and it _would still work_. The caller for RAM64 doesn't care how you use the "complete address". It just cares that you return the same thing for that address. -Fill.asm -: Figured out that my RAM16K implementation was wrong while working on this. The rough pseudocode would be: +## RAM512 + +I used the premise of the previous note and decided to index the RAM64 moduls by the LSB instead of the customary MSB. It still works :metal: + +## PC + +The Program Counter was tricky. I ended up using a Or8Way (instead of a `Or3Way`, which we don't have) along with `Mux8Way16` to pick the input for the register. Since the Mux has duplicate inputs at this point (supports 8, but we only have 3 special, and 1 neutral case) - this can be optimized by switching to a `Mux4Way16`, along with some other changes. + +## Fill.asm + +Figured out that my RAM16K implementation was wrong while working on this. The rough pseudocode would be: ```c int r0=*screen; while(true) { @@ -42,13 +47,16 @@ while(true) { So every "cycle" of the loop, we are coloring an entire row. The row is decided by R0, which is set to @SCREEN at the start. So if you press a key while we are on the middle of the loop (say 120th row), everything from that row onwards would get painted in black, and then the loop resets r0=\*screen once we cross the limits. The next iteration of the loop then starts filling the white pixels we'd left in the previous iteration. I kept the smallest paint unit as the row, but it doesn't really matter that much. The only difference is that I'm reading kbd a total of 256 times to paint the screen. Reading once per register also would work, and reading once per "screenfill" would also work. But that changes the 'delay' b/w your keyboard press and the screen fill start. I thought per row was a good compromise. -Memory -: Started by using a Mux4Way16, but then decided against making the same mistake I did in PC. Switched to 2 Mux16s instead. +## Memory -CPU -: No tricks, fairly straight forward implementation of a few Mux atop the ALU. Things I missed on the first pass: +Started by using a Mux4Way16, but then decided against making the same mistake I did in PC. Switched to 2 Mux16s instead. + +## CPU + +No tricks, fairly straight forward implementation of a few Mux atop the ALU. Things I missed on the first pass: - Ensuring that control bits for writeM or jumps are _only_ set when instruction is C. This is just ANDing the relevant control bit with instruction[15] - `!ng != positive`. Zero being the exception. So `positive = !(ng | zr)`. I think the whole jumpToAddress bit calculation can be improved though. -Computer -: The hardest part about this was deciding what name to give to all the pins +## Computer + +The hardest part about this was deciding what name to give to all the pins