Memory address decoding, RAM, ROM, and CF card part & symbol

The 8 bit computer has a 16bit memory addressing. It is segmented into 4 blocks. From 0x0000 to 0x7FFF is the 32K of RAM. From 0x8000 to 0x8007 is the CF card/external storage segment. From 0x8008 to 0xEFFF is reserved for now. 0xF000 to 0xFFFF is for the ROM/bootloader.

Memory address decoding

To only enable ROM/RAM/CF when its their memory address sections, I used the last 4 bits of the memory address. If A12-15 are all 1, then its the ROM thats selected. Since the ROM memory is 0xFXXX. The RAM is selected whenever the A15 is 0, since the 62256 only has 15bits of address. The CF is selected when a A15-A13 is 0b100, which is decoded using a 74LS138.

RAM

The RAM uses a 62256 ram chip. The 62256 has common I/O so the 74LS245 transceiver is responsible for both RAM to bus and bus to RAM (ie STA instructions). I’ve decided to not include manual programming of the RAM unlike Ben Eater’s original design. It was simply because it would make the modules way too big.

ROM

The ROM uses a 28C64 for now. Its more for the bootloader/storing programs. Its buffered to the bus using a 73LS245.

CF card

The CF card section is for programs, it works on sectors, so the bootloader has to copy the code from the CF into the memory in order to work. Currently I found a CF card part and made a symbol for the CF card reader.