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fix(nemu): encode trap as 0x0005006b so rs1 is a0 - #75
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The helpers and linux/hello already put the trap code in a0. NEMU always reads gpr[10] (a0), so `.word 0x0000006b` still works there because its decode mask does not require rs1=a0. NutShell and XiangShan decode this custom trap as I-type and take the argument from rs1. With rs1=x0 they see 0, so GOOD_TRAP, timer disable (0x100), and profiling notify (0x101) become an unexpected halt with code 0. Use 0x0005006b in nemu-trap, nemu-exec, and linux/hello so rs1 names a0.
poemonsense
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Sep 20, 2026
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Should we fix the nemu decoder as well?
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The helpers and linux/hello already put the trap code in a0. NEMU always reads gpr[10] (a0), so
.word 0x0000006bstill works there because its decode mask does not require rs1=a0.NutShell and XiangShan decode this custom trap as I-type and take the argument from rs1. With rs1=x0 they see 0, so GOOD_TRAP, timer disable (0x100), and profiling notify (0x101) become an unexpected halt with code 0.
Use 0x0005006b in nemu-trap, nemu-exec, and linux/hello so rs1 names a0.