Screen slip was a 'feature' used on older microcomputers. I used it on Apple II machines; it may have worked on others. In brief, it was a technique for producing semi-random, flashy displays on the machine's video output.

The Apple II had multiple types of screen buffer - LORES, or Low Resolution Graphics; HIRES, or High-Resolution graphics, and text. Although all were just designated memory pages, the display hardware would treat the information contained in them differently. In the LORES mode, a byte contained a colormap value, which painted a large (8x8 pixel?) block at an indexed location on screen. In HIRES mode, a byte contained a packed bitmap for 8 single pixels on/off states and (optionally) color; and in text mode, the byte was translated to an ASCII bitmap.

Note: It's been a while, and I probably made mistakes above. Correct away.

In any case, it was possible to shift the machine from displaying one mode to another by simply POKE-ing a memory location which controlled the current display mode. The Apple II's 6502 CPU could, if the program was put in in machine code, POKE this location faster than the vertical retrace cycle of the machine. The resultant multiple changes of display mode would cause the display to 'freak out' and display anything from snow to a flickering montage of the contents of the various display pages.

My favorite use of this was to sit down at a vacant Apple II, drop into the mini-assembler, and type in a line of hex codes (which I can no longer remember) which would:

  1. Change the reset vector so that hitting reset just did a JMP to the first POKE (really STX) instruction of the line
  2. POKE the display mode location to HIRES
  3. POKE the display mode location to TEXT
  4. JMP to the first POKE

...and then execute it and walk away. The Apple would go into a freakout (sometimes even with audio as the signals leaked into the audio hardware) and hitting reset (RESET on the Apple II was an INT, not a NMI) wouldn't do anything. You had to power it down.

I once came across an Apple II that would go right into an infinite reset cycle when powered up. Upon careful examination, I found that some clever chap had wedged a paperclip into one of the game controller pin sockets and into the composite video ground. The result of the short was, apparently, a constant low voltage on the RESET bus of the keyboard. Heehee.

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