FA-89732 / Instruction set emulation / Member archive

Writable CSR range treated as read-only · case 02

Writes to CSRs whose top bits are 10 trap as illegal.

Member previewVariant 2 · 3 implementations · 11 checks per implementation

Case contract

Input [priv, csr, op, rs1, value, csrs] with priv 0 (U), 1 (S) or 3 (M) and csrs mapping decimal-string CSR numbers to values. Unknown CSR -> 'illegal'. Bits 9:8 of the CSR number give the lowest privilege allowed; bits 11:10 equal to 3 mark it read-only. csrrw/csrrwi always write; csrrs/csrrc/csrrsi/csrrci write only when the rs1 field (register index, or the 5-bit immediate for i-forms) is nonzero. Writing a read-only CSR -> 'illegal'. Operand: the register value, or the rs1 field itself for i-forms. New value: rw -> operand, rs -> old | operand, rc -> old & ~operand. Return ['ok', old, new (old if no write)].

Why this case matters

Privileged-architecture emulation must reject illegal CSR accesses exactly, since operating systems probe CSRs and rely on traps.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
machine counter is writable	`"illegal"`	`["ok", 50, 4]`	Failed

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