A downloadable tool
Sky In-Home Service Ltd has kindly given its permission for the redistribution of its copyrighted material but retains that copyright.
Use of this software in unlicensed hardware is prohibited.
The Better Uncommented Spectrum ROM Assembly (BUSRA) is Z80 source code that can be used to build a copy of the original unmodified ZX Spectrum ROM. It compiles without modification with Pasmo (http://pasmo.speccy.org/), a cross-platform Z80 cross-assembler. Using macros, BUSRA implements the X80 FPU instruction set. X80 instructions are used by the ROM's internal floating point calculator routines. A language definition file that includes Z80 and X80 instructions (x80.xml) is provided for use with Notepad++. Using the X80 instruction set makes it easy to use the ROM floating point routines in your own code. A comprehensive definitions file almost entirely eradicates 'magic numbers' from the source, making it easy to read, even without comments.
For further information on the ROM, Source Solutions recommends The Complete Spectrum ROM Disassembly (Logan & O'Hara, 1983).