The 8-bit ALU requires may internal control signals. This simple two-level AND/OR circuit produces all of those signals, from a four bit input which corresponds to the 4-bit operand used in all of the instructions which use the ALU. For any of the 16 operations, this logic determines which parts of the ALU will process the 8-bit register data. To understand why this circuit asserts each signal, please see the detailed description of the ALU circuity works.

Schematic Drawing

OSU8: Simple 8-Bit Microprocessor Design; Paul Stoffregen
http://www.pjrc.com/tech/osu8/sch/alu_dec.html
Last updated: February 24, 2005
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