Reading HP67 ROMs

[teenix]

I am unsure of the ROM address decoding. I imagine they read the 12 bits on the Is bus and react accordingly. However, to access the full address space, which is $0000 to $17FF, there needs to be a 13th bit. It is feasible then that when a Bank Switch instruction is issued, one or more of the ROMs detect this instruction and becomes active for address above $0FFF. The ROMs will have to be mask programmed to respond or not respond to the instruction, the same as they will be mask programmed to occupy a specific 1K address space. This applies to ROM 0 too which also decodes the LED display for the anode driver.

The ROMs must also detect when an address is between $0000 and $03FF which disables the ROMs in the upper bank and allows the ROMs in the lower bank to respond. This is probably the reason why there is no mapped ROM space between $1000 and $13FF.

The HP-97 is the same except that with an extra ROM chip, the address range extends to $1BFF to cater for the printer.

New Classic Notes uploaded with some HP-67 bus info at the end.

cheers

Tony