Decodey spec - Atmelfan/C204 Wiki

Inputs:

  • instruction[15..0], instruction to be executed

Outputs:

  • asel[3..0]: selects A register
  • bsel[3..0]: selects b register
  • intermediate[7..0]: intermediate data
  • muxsel: selects between A register or intermediate data
  • op[3..0]: ALU operation
  • extended: Fetchy should ignore next word

Decoding:

1 instruction = mppp-iiii-aaaa-bbbb  

asel = aaaa  
bsel = bbbb  
intermediate = iiii & aaaa  
op = 0ppp  
muxsel = m  
extended = '1' when ppp = JMP else '0'
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