Editing Acorn Electron (section)

====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,<ref name="electronuser198903_firetrack">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume6/Electron-User-06-06/page/n4/mode/1up | title=Firetrack speeds in for the Electron | magazine=Electron User | date=March 1989 | access-date=28 February 2021 | volume=6 | issue=6 | page=5 }}</ref> exploits a division in the way the Electron handles its display{{snd}} 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 display. By using the palette to mask the top area of the display and taking care about when it changes mode it can shift the continuous graphics at the bottom of the display down in two pixel increments because the internal display counter is not incremented on blank scanlines during non-continuous graphics modes.<ref name="firetrack">{{ cite web | url=http://electrem.emuunlim.com/techinfo.htm#firetrack | title=Vertical bipixel hardware scrolling | website=Acorn Electron Technical Documentation | date=15 August 2003 | archive-url=https://web.archive.org/web/20060206155259/http://electrem.emuunlim.com/techinfo.htm#firetrack | access-date=5 March 2021 | archive-date=6 February 2006 }}</ref>