Z80 ASM Optimization Tricks - v2.1 (by Adam Ziemba, 2006) https://www.ticalc.org/archives/files/fileinfo/285/28502.html
Z80 Optimization Technies v1.1 (by Christopher Tremblay, 2000), https://www.ticalc.org/archives/files/fileinfo/108/10821.html
Z80 Optimization https://wikiti.brandonw.net/index.php?title=Z80_Optimization
Zeda/Z80-Optimized-Routines https://github.com/Zeda/Z80-Optimized-Routines
Z80 instruction set https://clrhome.org/table/
Retro Programming (by John Metcalf) http://www.retroprogramming.com/
Tutorials on z80-Heaven http://z80-heaven.wikidot.com/system:tutorials
Tips for golfing in Z80 or SM83 machine code https://codegolf.stackexchange.com/questions/230042/tips-for-golfing-in-z80-or-sm83-machine-code/
Twitter bot for BASIC and Assembler programming online: https://twitter.com/ZxSpectrumBot1
Spectrum Computing forum tread: https://spectrumcomputing.co.uk/forums/viewforum.php?f=6
How to Write Spectrum Games v1.1 (by Jonathan Cauldwell): https://jonathan-cauldwell.itch.io/how-to-write-spectrum-games
First Steps in Z80 Assembly Language v1.2.4 (by Darryl Sloan): http://ped.7gods.org/z80.pdf + YouTube (4 videos)
Z80 Assembly Language Subroutines (by Lance A. Leventhal and Winthrop Saville, 1983): http://www.bitsavers.org/pdf/osborne/books/Z80_Assembly_Language_Subroutines_1983.pdf
MSX Assembly Page http://map.grauw.nl/articles/
Z80 Optimisation Tricks
8 videos - Z80 Assembly Language for the ZX Spectrum Tutorial
3 video lessons - Learn Z80 Assembly 2021 Edition
several video lessons - Zilog Z80 Part X
SjASMPlus, command line z80 assembler: download + documentation
UnrealTSL emulator https://github.com/tslabs/zx-evo/tree/master/pentevo/unreal
SpecEmu emulator (for snow/rain "enable ula snow") http://www.emucr.com/2022/02/specemu-v32-build-20220221.html?m=1
List of all actual ZX Spectrum emulators https://github.com/redcode/ZXSpectrum/wiki/Emulators
GFX extractor: http://fight.pp.ua/examples/zx-extractor2
53c Chunkypaint: https://53c.nw-lab.com
Stellarpaint: https://irrlichtproject.de/stellarpaint
8x8 fonts collection: https://www.jimblimey.com/blog/24-zx-spectrum-fonts/
yet another 8x8 fonts: https://www.jimblimey.com/blog/32-more-zx-spectrum-fonts/
and even more 8x8 fonts: https://damieng.com/typography/zx-origins/
Russian WIN1251 8x8 font: http://electronics-and-mechanics.azm.su/page59.html
ZX Spectrum ROM collection http://zxspectrum.it.omegahg.com/_SIN.htm
Screen Optimizer v4.2 by PSV (1994, ZX) https://zxart.ee/eng/software/tool/graphics/visualscreen/screen-optimizer/screen-optimizer/
| Screen Visual Optimizer 1.0 by LFG (2000, ZX) https://zxart.ee/eng/software/tool/graphics/visualscreen/screen-visual-optimizer |
Screen Optimizer v1.0e by g0blinish, Den Popov (2016, Win) https://vtrd.in/pcutilz/SCROP10E.zip
OpAL v1.0 by Lethargeek (2017, Win) https://vtrd.in/pcutilz/OPALV1_0.zip
ZX Spectrum SCREEN$ Optimizer by HeartWare (2022, Win) https://www.facebook.com/groups/specnext/?multi_permalinks=1775766416114004
Basinc (Paintbox's "Optimise for tape saving") https://github.com/ref-xx/basinc
scr2AnimGIF (-opt switch) https://github.com/ZXDunny/scr2AnimGIF