The barrel shifter
The barrel shifter, which performs binary shifts, is another interesting component of the ARM1.
Most instructions use the barrel shifter, allowing a binary argument to be shifted left, shifted right, or rotated by any amount (0 to 31 bits).
While running the simulator, you can see diagonal lines jumping back and forth in the barrel shifter.
The diagram below shows the structure of the barrel shifter.
Bits flows into the shifter vertically with bit 0 on the left and bit 31 on the right. Output bits leave the shifter horizontally with bit 0 on the bottom and bit 31 on top. The diagonal lines visible in the barrel shifter show where the vertical lines are connected to the horizontal lines, generating a shifted output. Different positions of the diagonals result in different shifts.
The upper diagonal line shifts bits to the left, and the lower diagonal line shifts bits to the right. For a rotation, both diagonals are active; it may not be immediately obvious but in a rotation part of the word is shifted left and part is shifted right.
Structure of the barrel shifter in the ARM1 chip.
Zooming in on the barrel shifter shows exactly how it works. It contains a 32 by 32 crossbar grid of transistors, each connecting one vertical line to one horizontal line. The transistor gates are connected by diagonal control lines; transistors along the active diagonal connect the appropriate vertical and horizontal lines.
Thus, by activating the appropriate diagonals, the output lines are connected to the input lines, shifted by the desired amounts.
Since the chip’s input lines all run horizontally, there are 32 connections between input lines and the corresponding vertical bit lines.
Details of the barrel shifter in the ARM1 chip. Transistors along a specific diagonal are activated to connect the vertical bit lines and output lines. Each input line is connected to a vertical bit line through the indicated connections.
