🚀 go-pugleaf

Newsgroups: comp.sys.amiga.tech
Subject: Re: Retargetable graphics. Temporary Stop-gaps.
Summary: 
Expires: 
References: <12437@life.ai.mit.edu> <16612@cbmvax.commodore.com> <7307@sugar.hackercorp.com>
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Reply-To: dale@boing.UUCP (Dale Luck)
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Distribution: 
Organization: Boing, Milpitas, Ca.
Keywords: rtg,iff,24bit,video hardware

In article <7307@sugar.hackercorp.com> peter@sugar.hackercorp.com (Peter da Silva) writes:
>
>The BitBlt routines will, of course, have to emulate the blitter. Since the
>68020 has the horsepower to beat the blitter (according to Henry Spencer, who
>is a pretty reliable source) that's not such a great hardship.

The 68020 does not have the horsepower to beat the blitter given the 16 bit
bus and the memory access technology. And even if they were the same, the 020 would
need to be at least twice as fast as the blitter to get the same effective
throughput on a 1 bitplane system. If you realize that many operations require
both actual raster operations as well as non-raster operations, then the
fact that the blitter runs in parallel to the cpu sometimes makes the operations
instantaneous. (No time at all)  The blitter is able to shift two sources, mask one
of those sources,and then combine this with a third source do zero detection, all
in a way to use 100% of the available memory bandwidth. At the same time it
keeps track of 4 address registers, incrementing to the next memory locations
and checking for end of line and rectangle conditions. Applying modules when
necessary.  Amd no empty memory cycles.
A cycle to the blitter is 280nsec. To do the equivalent operation in the cpu
would take several instructions.

Two come anywhere near this would require a minimum of a 32 bit data bus as well
as timing on the bus that matches the blitters own speed.

>
>At least any such RTG interface should provide the same basic interface as
>graphics.library. Why even consider anything else?

The basic graphics.library is a very complex interface that includes some
very amiga specific concepts such as Sprites- Real and virtual, copper lists,
blitter objects-(beam synced and normal), animation objects,
multiple viewports, changing resolutions, colormaps all 'multitasking' the
monitor.  And finally an 8 bit per color allocation in the RastPort. We can
sort of get around this be creating a pen lookup table, so that it actually
uses another 32 bit location to fetch the actual pen number. But only 8 bytes
were allocated for minterms. Minterms per bitplane do not always make sense
in a chunky mode display. Things like these implementation details need to
be hid a bit.

>

Dale Luck
-- 
Dale Luck     GfxBase/Boing, Inc.
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