Monday, 8 December 2014

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.

Final Results

Congratulations to all who entered, especially Allan Høiberg and Introspec Zx who both discovered a 15-byte solution. The final results are as follows:

Allan Høiberg15
Introspec Zx15
Jim Bagley16
Paul Rhodes16
Krystian Włosek16
Tim Webber16
Steve Wetherill16
John Young16
Simon Brattel16
John Metcalf16
Dariusz EM17
Chris Walsh23

Winning Entries

Allan was the first to discover a 15-byte solution:

                LD BC,22272
                LD A,85
LoopB:          BIT 6,B
                RET Z
LoopC:          DEC C
                LD (BC),A
                JR NZ,LoopC
                DJNZ loopB

Introspec found a 15-byte solution with only one loop:

                ld hl,16384+6143
filloop5:       ld a,h
                sbc a,a
                xor %01010101
                ld (hl),a
                dec hl
                bit 6,h
                jr nz,filloop5

My own attempts all fell short at 16 bytes:

                ld hl,22528-256
                ld bc,24*256+170
fill:           dec l
                ld (hl),c
                jr nz,fill
                rrc c
                dec h
                djnz fill

Entries are archived on John Young's website. Thanks to everyone who entered or otherwise supported the challenge. :-)

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