least 10 KB was taken up by the display buffer in contiguous display modes. Although programs running on the BBC Micro could use the machine's 6522 chip to trigger interrupts at certain points in the update of each display frame, using these events to change the palette and potentially switching all colours to black, thus blanking regions of the screen and hiding non-graphical data that had been stored in screen memory, the Electron lacked such hardware capabilities as standard. However, it was possible to take advantage of the characteristics of interrupts that were provided, permitting palette changes after the top 100 lines of each display frame, thus facilitating the blanking of either the top 100 or bottom 156 lines of the display. Many games took advantage of this, gaining storage by leaving non-graphical data in the disabled area. Other games would simply load non-graphical data into the display and leave it visible as regions of apparently randomly coloured pixels. One notable example is Superior Software's Citadel. Although page flipping was a hardware possibility, the limited memory forced most applications to do all their drawing directly to the visible screen, often resulting in graphical flicker or visible redraw. A notable exception is Players' Joe Blade series. Tricks Firetrack: smooth vertical scrolling Although programs can alter the position of the screen in memory, the non-linear format of the display means that vertical scrolling can only be done in blocks of 8 pixels without further work. Firetrack, released on a compilation by Superior Software, exploits a division in the way the Electron handles its display of the seven available graphics modes, two are configured so that the final two of every ten scanlines are blank and are not based on the contents of RAM. If 16 scanlines of continuous graphical data are written to a character-block-aligned portion of the screen then they will appear as a continuous block in most modes but in the two non-continuous modes they will be displayed as two blocks of eight scanlines, separated in the middle by two blank scanlines. In order to keep track of its position within the display, the Electron maintains an internal display address counter. The same counter is used in both the continuous and non-continuous graphics modes and switching modes mid-frame does not cause any adjustment to the counter. Firetrack switches from a non-continuous to a continuous graphics mode part way down the