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[[File:Quadra 900.jpg|thumb]]
 
[[File:Quadra 900.jpg|thumb]]
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===Video===
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Obviously every display made by every 3rd party monitor vendor can't be
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supported by the onboard video, but the Quadras do support a much wider
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range of displays at a higher level of performance than any previous
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Macintosh.  The Quadra 700 and 900 support pixel depths ranging from 1 to
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32 bits per pixel (bpp), Apple displays ranging from the 512 x 384 12-inch
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color monitor through the 1152 x 870 21-inch color monitor, pixel clocks
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ranging from 12 to 100 MHz, and a variety of industry standards such as
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VGA, SVGA, NTSC, and PAL.  The Mac Quadra video port produces RS-343 RGB,
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and also provides horizontal, vertical and composite sync outputs. 
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Composite or S-video output is not provided, but can be accomplished by
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use of an external RGB-to-composite encoder.  The Quadra 700 and 900 also
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support Apple convolution for flicker-reduction on interlaced displays
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(i.e., NTSC and PAL) at up to 8 bpp.  The Mac Quadras automatically detect
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the type of display attached to the video connector via 3 'sense' pins on
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the video connector.  Depending on the wiring of these 3 pins, software in
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ROM configures the video hardware for one the supported display types.  (A
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full description of sense pin wiring and supported display types is in the
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2nd article.)
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The Quadra 700/900 provide the highest built-in video performance of any
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Macintosh CPU to date.  In a (very) simplified graphics model, we could
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say that performance depends on two main factors: processor horsepower and
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the bandwidth the processor has into frame  buffer memory.  These
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machines already have a fast processor - the 68040 - which runs standard
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32-bit QuickDraw.  To provide high bandwidth into frame buffer memory,
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dedicated video RAM (VRAM) was used for the frame buffer, and that VRAM
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was placed directly on the 68040 processor's local bus.  This provides the
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68040 the same access time into frame buffer memory that it has into main
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system RAM.  (Transfer rates can exceed 40 MBytes/sec.)  In addition,
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memory options such as fast page mode are supported, which can improve
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graphics performance for operations such as scrolling,
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offscreen-to-onscreen pixmap transfers, etc.
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In a number of cases the design was optimized for high performance over
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low cost.  A good example of this is 32 bpp operation on Apple's standard
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13-inch RGB monitor at 640 x 480 resolution (and this also applies to VGA
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and NTSC), which is probably the most common color monitor in use on the
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Macintosh.  The actual number of memory bytes needed to support 24 bpp is
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640 x 480 x 3 = 921,600.  This would seem to fit within 1 MByte of memory
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(as is the case with the Apple 8*24 video card), but the Quadras actually
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require 2 MBytes of VRAM for this mode.  The 8*24 card supports 24 bpp at
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640 x 480 by using a storage mode called 'chunky planar' to fully utilize
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all its 1 MByte of VRAM.  However, this results in having to perform 3
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separate memory accesses for each 24-bit pixel read from or written to the
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frame buffer.  (This is done in hardware so software only performs a
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single read or write.)  On a NuBus video card, this inefficiency is
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partially masked by the synchronization delays which occur at the
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processor-bus/NuBus interface.  However, when frame buffer memory is
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placed directly on the processor bus, this approach results in a nearly 3X
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performance degradation.  This was judged unacceptable for the Quadras. 
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Each 24-bit pixel occupies one longword (4-bytes) in VRAM, so the Quadras
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actually provide 32 bpp for the 640 x 480 resolution.  This pushes the
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memory requirement for this mode over the 1 MByte boundary (640 x 480 x 4
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= 1,228,800 bytes).  Performance is improved still more by another frame
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buffer architectural feature.  Frame buffer memory in the Quadras is
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organized into 4 'banks' of 512 KBytes per bank.  As mentioned earlier,
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Quadra VRAM can operate in fast page mode.  In addition, each bank of VRAM
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operates in fast page mode independently of the other 3 banks.  This
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causes the number of in-page 'hits' to increase, and thus improves the
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effective bandwidth into the frame buffer.  Also, at 32 bpp, 640 x 480
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resolution, each row is set to 4096 bytes, or 1024 32-bit pixels.  Each
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successive row is assigned to a different VRAM bank (modulo 4, of course).
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This memory organization improves performance during certain commonly
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performed graphics operations such as vertical scrolling.
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In any design there are a number of tradeoffs to be made, and this is
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certainly true for the frame buffer in the Mac Quadra machines.  While the
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video does operate at 32 bpp on up to 16-inch displays, it does not
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support 21-inch displays at this pixel depth since this would have
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significantly raised the cost of the motherboard.  (Memory capacity and
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bus bandwidths would essentially have to double, and this would be
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expensive.)  It does support NTSC and PAL timing, but does not provide a
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composite video output.  While it is much faster than any non-accelerated
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video card, there are accelerated video cards that are faster (and much
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more expensive, too, by the way).  A separate graphics processor was not
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added primarily for cost reasons.  However, a graphics processor such as
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the 29000 RISC chip on the 8*24GC card can only speed up the graphics
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operations that it was designed to know about.  If an application program
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bypasses QuickDraw (which is what most Mac graphics accelerators are
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designed to speed up), a graphics accelerator will not improve
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performance, and can actually cause a performance degradation.
    
=See Also=
 
=See Also=

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