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From: ard@siva.bris.ac.uk (PDP11 Hacker .....)
Newsgroups: alt.sys.perq
Subject: Another PERQ filk
Message-ID: <16AUG199400401741@siva.bris.ac.uk>
Date: 15 Aug 94 23:40:00 GMT
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Organization: University of Bristol Physics Department
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Ever wondered just how the PERQ raster-op logic operates? Well, This filksong
just might help....

[Appologies to clementine :-)]

In the wierd PERQ CPU board
Released in seventy-nine
Live some digital logic circuits
Graphics bitmaps to combine

Chorus:
  Raster Ops Now! Raster Ops Now!
  Through the shift and to combine
  Watch the screen image updating
  Through the raster op pipeline

The fifo is loaded straight from memory.
That's the first stage of our line.
Clocked in by the controlling logic
Quadwords loaded all the time.

Source data flows out of the fifo
To the shifter which shall align
The edges of updated regions
To make them easy to combine.

But each word of the destination
With 2 source words may combine
So the last word out of the fifo
is latched in the semi-pipeline

Take thirty-one bits of the source words
From the last stage of the line
The shifter shall select just sixteen
To feed on and thus combine

The destination words from memory
Through a single stage pipeline 
That's all that's needed before they enter
Proms and thus with source combine

The Proms take source and destination
A new bitmap to combine
Edges of the updated region
The mask word shall now define

There is one more D-type register
The last stage of our pipeline
To send the data back to memory
Two clocks later - just in time (oooops Tine :-))

The source and destination bit fields
The shift value shall define
Subtracted by a control PROM 
Thus the 2 bitmaps align

The width of the updated region 
With the dest we now combine
This determines the edge positions
And the mask we can devine

A pair of small finite state machines 
Shall ensure this runs to time
Control signals thus generating 
For the raster op pipeline



-tony

Bristol University takes no responsibility for the views expressed in this
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