July 18, 2013 AT 5:31 pm

Reverse-engineering the flag circuits in the 8085 processor

Alu-Details

Ken Shirriff’s blog: Reverse-engineering the flag circuits in the 8085 processor.

Processors all have status flags to keep track of conditions such as a zero value, a carry, or a negative value. Whenever you write a loop or conditional, these flags ultimately are in control. But how are these flags implemented in the chip’s silicon? I’ve reverse-engineered the flag circuits in the 8085 microprocessor and explain what is really going on.
The photograph below is a highly magnified image of the 8085′s silicon, showing the relevant parts of the chip. In the upper-left, the arithmetic logic unit (ALU) performs 8-bit arithmetic operations. The status flag circuitry is below the ALU and the flags are connected to the data bus (indicated in blue). To the right of the ALU, the control PLA decodes the instructions into control lines that control the operations of the ALU and flag circuits.

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