3 ms·
Yes, the HP voltmeter used two 74LS181 ALU chips so it could do error and scaling calculations. The ALU was accessed through four I/O ports: two for the argume
by kens 6y ago
Yes, the HP voltmeter used two 74LS181 ALU chips so it could do error and scaling calculations.
The ALU was accessed through four I/O ports: two for the arguments, one for the operation and carry-in, and one to read the result. It wasn't memory-mapped, but I/O mapped since the Nanoprocessor didn't have memory operations (except reading instructions from ROM).
Instead of memory-mapped I/O, the Nanoprocessor had I/O-mapped memory. The real time clock module had 256 bytes of RAM that were accessed through I/O ports.
- monocasa 6y agoWhat's the distinction you're making between mmio and I/O mapped? That it only has absolute addressing? Or that it just calls it I/O?
- kens 6y agoMemory and I/O were separate spaces with separate pins and separate operations. The Nanoprocessor had 11 address lines for reading instructions from a 2K ROM. It had 4 I/O device select lines for accessing 15 I/O devices. So if you added RAM (as in the real time clock), the RAM was accessed through I/O instructions. You'd write the address to one port and read the data through another port. It ended up looking a lot like microcode, with memory accesses split into two pieces.
- deleted 6y ago[deleted]