SDCC's z80 crt0 sets SP=0x0000 and makes its first stack push at 0xFFFF.
The chip only mapped RAM at 0x8000-0xBFFF (0xC000+ was MMIO/ignored), so the
stack landed on unmapped memory and a plain C program crashed inside crt0 —
before main — which is why z80-led-chaser-c compiled but drove nothing.
Extend RAM to cover 0x8000-0xFFFF (32 KB) with the MMIO window 0xC000-0xC0FF
carved out and checked first, in scripts/make-z80-cpu.py + regenerated
z80-cpu.c. Now SDCC's default stack works and "write C from scratch, click
Run" just works — no manual `LD SP` needed (dropped from chaser.c). Bumped
the chip WASM initial memory to 4 pages to hold the larger RAM buffer. Larson
(asm, SP=0xBFFF, LED at 0xC000) is unaffected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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