RISC-V soft core 10: What does a multiply cost?

Mon, 19 Oct 2026 00:40:00 +1100

What does a multiply cost?

Part 10 of the RISC-V soft core series. Previously: First C on the core.

The first C program had a line in it that looks harmless: a * b. On a modern laptop that is one instruction and a few clock cycles. On this core there is no instruction for it. The core is RV32E with no M extension, so no mul, no div and no rem. The compiler turns each *, / and % into a call to a library routine, a loop of shifts and adds that runs as ordinary code.

RISC-V soft core 9: First C on the core

Mon, 19 Oct 2026 00:35:00 +1100

First C on the core

Part 9 of the RISC-V soft core series. Previously: Sleeping, and measuring it.

Back in part 0 I said the reason for choosing RISC-V over a machine of my own design was that the compiler already exists. It was a promise I had not yet kept. Everything the core had run so far was assembly I wrote or generated for the tests.

This post is the first time a compiler’s output ran on it. It is also where the thing I built stops being a toy and starts being something you could write programs for.

C on Ampsand

RISC-V soft core 10: What does a multiply cost?

RISC-V soft core 9: First C on the core