Z80 Size Programming Challenge #1

A few days ago I issued a Z80 programming challenge for the ZX Spectrum:

Something simple for the first challenge. Write the shortest code to fill the screen with a chequerboard pattern of 1 pixel squares. No RAM/ROM other than the 6144 byte bitmap screen memory should be written to.

Target: under 25 bytes.

1. Your program shouldn’t rely on the initial contents of registers.
2. Programs must return. The RET instruction is included in the size.
3. So everyone has a fair chance comment with the code size not code.
4. There are no prizes, just the chance to show off your coding skill.

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thanks to JM @ http://www.retroprogramming.com/