Hp45 microcode



#2

Whoever has problems squeezing the microcode instructions in the rom of a ucontroller, can try the following to save space:
There are 1024 possible instructions, but only about 526 are used.
The Roms 1,5 and 6 use a similar subset of only 254 instructions. So you number the 254 instructions from 0 to 253 and store them instead of the original 10bit instructions. Next you have to build a translation table which translates the 8 bit number to the original opcode.

If you only have the choice between 8 and 16 bit words, you can save space:

before: 8Roms*256Instructions*16Bit

after: 5ROMS*256Instructions*16Bit+
3ROMS*256Instructions*8Bit+
1Translation*254Instructions*16Bit


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