In the previous post I mentioned at the end that the BIOS, running on the ATMega328p, was only capable of communication one way. There are a couple of mailbox memory locations that are used to pass keystrokes to the 8-bit CPUs (Z80 or 6502), yet no way of receiving data back, other than an acknowledgement that the keystroke has been processed. In order to make the BIOS a bit more BASIC-friendly, I needed to add …
One of the demos I’ve carried across nearly all the platforms I’ve worked on is the rotating cube demo, typically with back-face culling (hidden line/face removal), perspective, and fast integer maths. The code has been ported to the Gameboy, CPC, PC, Atari ST and Scratch. On more capable systems I usually add filled-in polygons with simple texture mapping and Gouraud shading I first wrote this demo around 30-odd years ago for the Spectrum, and I …
The Spectrum screen handling is not the most straightforward and, with lack of hardware sprites, scrolling, and an awkward layout, can be intimidating at first. This post details how I managed to scroll a 24×24 character block of tiles on the Spectrum within one VBLANK interval. The scroll routine employs a number of tricks to achieve this by: Using the stack pointer (SP) to fetch tile data and push it to the screen. Using self-modifying …
