nx_emu_cortexm.nx

nx_emu_cortexm.nx -- sovereign ARMv7-M / Cortex-M (Thumb-2) interpreter (NX-EMU). Little-endian, MIXED 16/32-bit instructions (32-bit iff hw1[15:11] in {11101,11110, 11111} i.e. hw1>=0xE800), 16 GPRs + NZCV. Bare-metal: BKPT halts with result=r0 (no OS). NO qemu. license_tier: ORIGINAL 2026-09-03, ISA LANE. WHAT THIS REVISION ADDS AND WHY. The first cut decoded exactly the subset nxc2 cortex_m backend emits: push/pop, mov(hi), movs, adds/subs/adcs/sbcs (register form only), sub-sp, ldr/str, blx, movw, umull, mla, subw. Measured against the ARMv7-M Architecture Reference Manual that left the machine unable to run a PROGRAM rather than a straight line: * NO conditional branch at all (B-cond, encoding 1101 cond imm8) -- so no loop, no if, no early exit could ever execute. * NO unconditional branch (B T2, 11100 imm11). * NO CMP in either form, and only ONE flag (C). N, Z and V did not exist, so even a hand-written branch would have had nothing to test. * NO immediate add/sub (imm3 and imm8 forms), no logic (AND EOR ORR BIC MVN TST), no shifts (LSL LSR ASR ROR), no MUL, no byte/halfword access, no SP-relative or PC-relative load, no CBZ/CBNZ, no extends. An emulator that cannot take a branch cannot run a loop, and a loop is the first program anyone writes. So the gap was CLOSED rather than reported: this file now carries full NZCV, the ARM add/subtract-with-carry adder that derives all four flags from one place, the condition-code table, and the decode above. THE OTHER DEFECT FIXED HERE, AND IT IS THE DANGEROUS ONE: the step-budget path used to leave result at its initial 0 and halt. 0 is ALSO a legitimate exit status, so a runaway program and a program that exited cleanly with 0 returned the SAME VALUE. A non-completion that is indistinguishable from a real answer is worse than a crash. Non-completion now has NAMED sentinels, matching the rv64/mips64 siblings so one ruler can classify all of them: -1 UNSUPPORTED, -2 RAN-OFF-END, -3 FAULT. RAN-OFF-END is structurally unreachable in this interpreter (the loop can only exit through a halt) and that is stated rather than hidden -- the constant exists so the classifier third state has a name here too, not because this file can produce it. SCOPE, STATED SO NOBODY READS THIS AS COMPLETE: IT blocks (0xBF00), SVC (0xDF00), UDF (0xDE00), LDM/STM (0xC000), REV/REVSH, and every 32-bit Thumb-2 encoding outside {MOVW SUBW UMULL MLA LDR.W STR.W} are still UNDECODED and correctly report UNSUPPORTED. UDF is PERMANENTLY UNDEFINED by the manual, which makes it a stable negative control: nx_isa_cortexm_gate uses it to prove the refusal path fires.