LoPROC

The LoPROC is a single cycle 32-bit micro-processor with very low power consumption. The processor is optimized from the instruction encoding stage to reduce the size and increase the performance of the instruction decoder.

Instruction Encoding

There are 2 types of instructions:

1. N-Type - Normal Type: These are instructions performed on register/ memory values.
2. I-Type - Immediate Type: These instructions are performed on immediate values.

The values for the instructions are chosen such that the decoding logic is not complex.

N-Type

SRC2-sel will be 0 for N-type Instructions

I-Type

SRC2-sel will be 1 for I-type instructions

MODE:

1. 00 - Memory operations/ Jump/ Function calls
2. 01 - Shift/ Rotate Operations
3. 10 - Multiplication
4. 11 - ALU Operations

ALU

The ALU is based on the HACK ALU developed by Noam Nisan and Shimon Schocken.

PSW

1. INT: Sets when Interrupt occurs
2. SE: Sets when stack is empty
3. SF: Sets when stack is full
4. DBZ: Sets when there is a division by zero
5. Z: Zero - Sets when ALU output is zero
6. N: Negative - Sets when ALU output is negative
7. ODD: Sets when ALU output is odd
8. P: Parity flag – sets when ALU output has odd number of 1s
9. OV: Overflow – sets when overflow occurs
10. C: Carry – sets when there is a carry
11. User: User flag – can be defined by user
12. IE: Interrupt Enable – Needs to be set to enable interrupt
13. RS1, RS0: Register Bank Select – Selects one out of the four register banks
14. 2 flags are reserved

Multiplier

The multiplier runs on optimized add-shift algorithm. The add-shift algorithm checks all the bits and shifts when the bit is 1. This algorithm used encoder-decoder duo to check which bits are 1, and performs the shifts.

Shifter

The shifter is a fully combinational universal logic. It can perform all the rotation, shifting operations.