TMS9918A Video Upgrade: Part 2

This second installment involves some explanation of the changes to the operating system and how they have been implemented. By actually placing the contents of the ROM on EPROM, you can change any routine, implement extra commands outside of the 8K ROM and have BASIC control of your new routines. It is only necessary to know where the table of information is, that tells the operating system where to go to execute the BASIC command (next time). Since The TMS9918A VDP chip creates the display and has its own memory, a few Sinclair routines become unnecessary. These areas are precisely where the new instructions go to make our ZX/TS computers compatible with the VDP chip. To increase compatibility further, some other routines are “patched” into as well.