ZX Spectrum VC3-2023
Author: Trevor R.H. Clarke <retrev@csh.rit.edu>
Category: Christmas Challenge
System: ZX Spectrum 48k
Language: z80 macro assembler (SjASMPlus flavor)
Len source code: 6600
Len exe file: 49179
Len code only: 188
Instructions:
Use and ZX Spectrum 48k you'd like but you can find an online one here: https://jsspeccy.zxdemo.org
Click the Folder icon and select the .sna file. It should load and execute in a second or two.
Description:
It's a pretty simple program optimized for size not speed. I'm using some routines written by Deam Belfield,
to clear the screen and draw the characters in the correct place. I've rewritten them a bit to cut size down as much as possible.
The coordinates of the border asterisks are stored as bytes in the stack area. There's some buffer above it
to ensure we don't overwrite the code during routine calls.
Setup disables interrupts, loads the SP, and clears the screen.
The main loop copies the next coordinate from the stack, draws the line down and to the left, copies the coordinate
again and draws the down and right line, and finally pops the coordinate and draw the line that's up and right. If
we've hit the top of the stack, it's done and we halt, otherwise we loop and do it again.
There are some code docs explaining in greater detail.
Comments:
The executable code is entirely contained in the $8000 block of memory. You can subtract that from the last thing in the block (the top_of_stack) to
get the code size. This is easy to do in something like DeZog. The sna file format has a fair amount of overhead as it's a memory snapshot and saves
complete memory banks, but it's the most likely to work with an emulator. I've also included a .nex file for the ZX Spectrum NEXT which is a bit
more compact (16896) but fewer emulators support the format.