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Address generation

A project log for Kobold - retro TTL computer

A computer with 20 address bits, 16 bit instructions, video display, from just a few TTL and memory chips. Aiming for PDP11 instruction set.

roelhroelh 04/10/2019 at 14:150 Comments

For generating the memory address, the square inch processor had only a H-L register pair, that had to be reloaded each time when another address was needed. For the new processor, I want to have several addresses on standby, ready to be connected to the address bus when needed. I also want 20 bit addresses.

The 74HC670 seems to be very suitable for this. It has 4 latches of 4 bits each (and only 16 pins). If we use five of them, we have four 20-bit registers. 1-to-4 decoders for read and write are built-in ! Here are the internals:

Bits 1 to 4 will get or-gates or an adder to add the 4-bit word-displacement to the address. The lowest address bit, bit 0, is used to select upper or lower byte in a word.

So, the address generation takes only 6 chips !

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