BusSHR

Inputs

Outputs

How It Works

A right shift moves every bit of the bus one position to the right. The LSB (bit 0) falls off the edge and is lost, while a 0 arrives at the MSB (bit 7). Byte example: A = 00001100 (12) → Q = 00000110 (6).

This gives the operation its meaning: a right shift is division by 2 with the remainder discarded. The chain 12 → 6 → 3 → 1 → 0 shows how quickly the low bits burn away. In binary, shifts replace multiplication and division by powers of two — just as an extra zero on the right of a decimal number multiplies it by 10.

One nuance: BusSHR is a logical shift — a 0 is always written on the left. That is exactly what unsigned values need, but the sign bit is not preserved: an 8-bit two's-complement 11111111 (−1) turns into 01111111 (127).

8-bit Examples

Usage

In the ALU (level 13) a shift is a basic operation alongside AND, OR and addition. This is how the processor divides by 2, 4 and 8 without a dedicated division unit: three shifts in a row give division by 8.

The second role is bit extraction. In the Harvard architecture (level 16) an instruction fits in a single byte: the high 4 bits are the opcode, the low 4 bits the operand. Shift the instruction byte right by 4 to get the opcode; the remaining low bits are the operand. The same trick reads gamepad presses and switch states: shift right until the bit you need lands at position 0, then apply a mask with BusAND.