8. The Register Files and Bypass Network

Multi-Issue Pipeline

Fig. 8.1 An example multi-issue pipeline. The integer register file needs 6 read ports and 3 write ports for the execution units present. The FP register file needs 3 read ports and 2 write ports. FP and memory operations share a long latency write port to both the integer and FP register file. To make scheduling of the write port trivial, the ALU’s pipeline is lengthened to match the FPU latency. The ALU is able to bypass from any of these stages to dependent instructions in the Register Read stage.

BOOM is a unified, physical register file (PRF) design. The register files hold both the committed and speculative state. Additionally, there are two register files: one for integer and one for floating point register values. The Map Tables track which physical register corresponds to which ISA register.

BOOM uses the Berkeley hardfloat floating point units which use an internal 65-bit operand format (https://github.com/ucb-bar/berkeley-hardfloat). Therefore, all physical floating point registers are 65-bits.

8.1. Register Read

The register file statically provisions all of the register read ports required to satisfy all issued instructions. For example, if issue port #0 corresponds to an integer ALU and issue port #1 corresponds to memory unit, then the first two register read ports will statically serve the ALU and the next two register read ports will service the memory unit for four total read ports.

8.1.1. Dynamic Read Port Scheduling

Future designs can improve area-efficiency by provisioning fewer register read ports and using dynamically scheduling to arbitrate for them. This is particularly helpful as most instructions need only one operand. However, it does add extra complexity to the design, which is often manifested as extra pipeline stages to arbitrate and detect structural hazards. It also requires the ability to kill issued Micro-Ops and re-issue them from the Issue Queue on a later cycle.

8.2. Bypass Network

ALU operations can be issued back-to-back by having the write-back values forwarded through the bypass network. Bypassing occurs at the end of the Register Read stage.