halvblank

Halvblank, short for half vertical blanking interval, is a term used in video hardware and retro-computing contexts to describe a portion of the vertical blanking interval during which non-display memory updates or peripheral actions can be performed without visible effects on the image. The vertical blanking interval (VBLANK) is the time between the end of one frame’s active scan and the start of the next frame, when a CRT’s electron beam returns from the bottom to the top (or, in modern terms, when the display is not actively drawing). The halvblank concept splits this interval into subparts, allowing certain work to be done earlier or later in the blanking period.

In practice, halvblank scheduling is used to reduce memory bandwidth contention and to avoid tearing or glitches by performing framebuffer writes, buffer swaps, palette changes, or DMA transfers while the display is not actively fetching pixel data. The idea is to place updates in a window within VBLANK that is short enough to avoid timing conflicts yet long enough for reliable memory access. Some hardware descriptions and emulation code refer to a halvblank window or a HALVBLANK signal/flag, indicating the start or presence of a half-interval during the vertical blank.

Today, the strict need for halvblank scheduling is less common on modern GPUs and LCD panels, where vertical synchronization and buffering are handled automatically. However, the concept remains relevant in the study of vintage systems, low-level graphics programming, and emulation, providing a historical approach to timing updates with the display cycle.