-- card: 55604 from stack: in
-- bmap block id: 0
-- flags: 0000
-- background id: 2792
-- name: glossary 9
----- HyperTalk script -----
on closecard
  global maploc, cardloc
  put "yes" into line 9 of fld "footprints" of card "map"
  put word 1 of the short name of this card into maploc
  put word 2 of the short name of this card into cardloc
end closecard



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----- text -----
Macintosh Display Card 4•8 and 8•24

-- part contents for background part 4
----- text -----
Glossary

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----- text -----
VRAM (Video RAM)

-- part contents for background part 11
----- text -----
glossary9

-- part contents for background part 5
----- text -----

An exact pixel-map representation of the current state of the display is stored in RAM on the card.  This is the primary point of contention in most graphics-based video circuitry.  The reason for this contention is that the display must refresh completely every 1/67th of a second (minimum speed to avoid flicker), thus the circuitry driving the analog signal to the display must have constant, regular access to the pixel map or the image will fade or flicker on the screen.  

The key difference between regular RAM and VRAM is that VRAM is dual-ported.  This allows the CPU and analog display circuitry to have simultaneous access to the pixel map without contention.