rv32c: complete the Zcb extension (unblocks RP2350 flash/bootrom boot)#1
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rv32c: complete the Zcb extension (unblocks RP2350 flash/bootrom boot)#1prat96 wants to merge 1 commit into
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The decoder handled only c.lbu/c.lhu/c.not from Zcb. The RP2350 boot ROM is compiled with Zcb, so every flash binary aborts in early ROM with e.g. "Unknown compressed instruction: 0x9fe1" (c.zext.b) or "Unsupported Zcb instruction: 0x8c24" (c.sh). Add the rest of Zcb, decompressing each to an instruction the core already runs: c.zext.b->andi, c.sext.b/c.sext.h->Zbb, c.zext.h->pack(rd,rd,x0), c.not->xori, c.mul->mul, and the c.lh/c.sb/c.sh loads/stores. c.zext.w is RV64-only and left out. Adds src/riscv/rv32c-zcb.spec.ts (9 cases, including the two real bootrom encodings that originally blocked boot). Full suite stays green. Signed-off-by: Pratheek Balakrishna <pratheekb96@gmail.com>
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What
Completes RV32 Zcb compressed-instruction decoding in
src/riscv/rv32c.ts. The decoderpreviously handled only
c.lbu,c.lhu, andc.not; this adds the rest of the extension:c.lh,c.sb,c.shc.zext.b,c.sext.b,c.zext.h,c.sext.hc.mulEach is decompressed to an instruction the core already executes —
andi,sext.b/sext.h(Zbb),
pack rd,rd,x0(=zext.h),xori,mul(RV32M), andlh/sb/sh.Why
The RP2350 boot ROM is built with Zcb, so these encodings appear during early boot. With
them missing, any RP2350 flash binary aborts almost immediately — booting a real board's
firmware died at
Unknown compressed instruction: 0x9fe1(c.zext.b) inside the ROM, thenUnsupported Zcb instruction: 0x8c24(c.sh). Completing Zcb lets flash images boot throughthe ROM. (The
rp2350js/WIPREADME already listsRV32Zcb (lbu, lhu, not)as partial.)Tests
Adds
src/riscv/rv32c-zcb.spec.ts— 9 cases covering every new opcode, including the two realbootrom encodings (
0x9fe1,0x8c24) that triggered this.npx vitest runis green(355 passed, including the existing suite).
Notes
c.zext.wis RV64-only and intentionally not handled.decompress_rv32c_inst"cannot handle index" error include the raw encodingand PC, which is what made the missing instructions easy to spot.
Based on
rp2350js/WIP.