FDDD - redcode/Z80 GitHub Wiki

Platform: ZX Spectrum
Author: Woodmass, Mark
Release date: 2008-03-08 ^{(unreleased, provided by the author)}
Links: Discord post
Download: tap, asm _^(hosted) - zip _{^{(Discord post)}}

This test needs the "allRAM" mode introduced in the ZX Spectrum +3 model (+2A as well).

Test fills four 16 KiB banks with FD DD byte pairs, and does switch to "allram" mode creating solid 64kiB of FD DD FD DD ... block, executing the FD/DD prefixes infinitely, with interrupts disabled (IFF1=0 upon entering the allram mode).

Static (flashing) screen filled with FD DD bytes (vertical stripes with flashing bright cyan/white and magenda/cyan attributes), CPU not reacting to interrupt requests (also NMI), only reacts to reset or power cycle. The stack pointer is set into attributes area, so any change in the attributes signals incorrect Z80 emulation, allowing for some other code being executed.

This test was patched for variant FDDD2 which enters the allRAM 64 KiB FD DD block with interrupts enabled (IFF1=1).