Sinclair ZX Spectrum +3 Video RAM Bytes Allocation

This article details the memory specifications of the Sinclair ZX Spectrum +3, focusing on the bytes allocated for video display. It clarifies that the system lacks dedicated video RAM, utilizing main system memory instead. The text provides the exact byte count used for the screen display file and explains the technical architecture behind the graphics generation.

The Sinclair ZX Spectrum +3, released in 1987, utilized a unified memory architecture rather than separating system and video memory. Consequently, there were no dedicated video RAM chips installed on the motherboard. Instead, the Video Logic Array (ULA) accessed a specific portion of the main 128KB system RAM to generate the visual output displayed on a monitor or television set. This design choice was consistent across the ZX Spectrum range to keep manufacturing costs low while maintaining compatibility with existing software.

For the standard display mode, a total of 6912 bytes of system RAM were allocated for the screen display file. This memory allocation is divided into two distinct sections. The first section consists of 6144 bytes dedicated to the pixel bitmap, which defines the black and white structure of the graphics at a resolution of 256 by 192 pixels. The second section comprises 768 bytes used for color attributes, allowing for 8 colors and 2 brightness levels within each 8 by 8 pixel character block.

Although the machine possessed 128KB of total RAM, the memory mapping for the display remained consistent with earlier 128K models. The ULA contended with the Z80 CPU for access to this memory region, which could occasionally result in visual artifacts known as attribute clash during heavy processing. Understanding this 6912-byte allocation is essential for developers and historians analyzing the graphical capabilities and limitations of the Sinclair ZX Spectrum +3 hardware.