I declare that today, Nov. 19, 2025 is the 50th anniversary of BitBLT, a routine so fundamental to computer graphics that we don’t even think about it having an origin. A working (later optimized) implementation was devised on the Xerox Alto by members of the Smalltalk team. It made it easy to arbitrarily copy and move arbitrary rectangles of bits in a graphical bitmap. It was this routine that made Smalltalk’s graphical interface possible. Below is part of a PARC-internal memo detailing it.
BitBLT was implemented in microcode on the Alto and exposed to the end-user as just another assembly language instruction, alongside your regular old Nova instructions – this is how foundational it was. And since it was an integral part of the Alto, it enabled all sorts of interesting experimentation with graphics: user interfaces and human/computer interaction, font rasterization, laser printing… maybe a game or three…
See also XScreenSaver 's BlitSpin hack, which I wrote in 1992, based on SmallTalk code published in the August 1981 issue of Byte Magazine. (which you can see on page 188 thanks to the priceless treasure that is the Internet Archive.) One way of looking at it is that it uses a scratch bitmap as a carry bit, performing parallel math on the Z axis.
And it wasn’t just a gimmick; if you had BitBLT microcode, it was a very fast way of rotating a bitmap!
The details on BitBLT itself start on page 172. Circle 359 on inquiry card.
From jwz via this RSS feed