piece of iron - "SPRINTER-97": graphic accelerator.

  (C) Ivan Makarchenko, 1997.



         ACCELERATOR operation screen.


  The main idea of the accelerator in the possibility of 
using the maximum possible speed data transfers to video RAM. 

  At a frequency of 3.5 MHz of video RAM can be
information at a rate of 3,500,000 bytes per second. When the 
volume of Spectrum-ovsky screen 6.3K This speed allows, for 
example, to clear the entire Screen time: 2 milliseconds (1 / 
10 part of the screen cycle). Scrolling the entire screen is 
carried out 4 milliseconds (you need to read information from

screen and move it). If we use
such speed and in other operations, this
will give a huge increase in the dynamics of the screen
images.


  How does the processor Z80. He chooses his
command, decodes it, and then executes. That
would send a single byte on a normal frequency of 4 
microseconds is required (together with command transfer). One 
byte, the second, on the third .. It turns out, to forward the 
entire screen is required 25 milliseconds.



  What should I do? Turbirovanie processor is not omnipotent. 
You can increase the frequency to twice a three ... four is a 
big question. Let we increased the frequency of four times and 
speed Shipping has increased in much the same. Obtain time: 1 
microsecond per byte, 6.3 milliseconds to the entire screen. 
And this is the limit above which turbirovanie processor does 
not jump. 


  But the screen RAM allows you to work with more
greater speed. At a minimum frequency of 3.5
MHz maximum baud rate in half
exceeds the limit of the processor. And what if the RAM runs at 
a higher frequency? At 7, 14, 28 ... For RAM, these frequencies 
are real at Installation of the chip. Here

and gives the effect of accelerator on-screen operations.


  Accelerator layout is quite simple and the whole meaning
that at the time of applying the processor to the video RAM is 
not one, but several sample data as fast as possible for the 
RAM. The processor is just waiting. Information block is 
rewritten into the cache of the accelerator, and then 
Similarly, it can be transferred to another place the video 
RAM. And in the same block. The rate of exchange of information 
on the screen rises sharply! 

  In addition to these operations, cleaning and scrolling 
screen accelerator provides a few functions, but they are at 
other times. 


  A pilot scheme of the accelerator has already been 
implemented and tested. And here it must be said that's what. 
To achieve maximum acceleration, corresponding limit on the 
frequency of video RAM impossible due to the consistent 
performance of the processor and the accelerator. At the time 
of the accelerator processor waits for its end, and vice versa. 
But in a similar mode of operation achieved acceleration

work with the video several times.


  Time to full image Spectrum-ovsky screen when it has 
previously been loaded into the RAM accelerator is 3 
milliseconds. This time is virtually independent of mode 
processor TURBO / no-TURBO, because that most of the time 
running the accelerator. 


  For new modes this time will be longer, but
it is in any case is less than the time at which the transfer 
was made images on the screen in the old Spectrum-e.



  For a graphic image to 320x256 pixels
16 colors, this time is approximately 15 milliseconds. (For a 
conventional ZX-Spectrum while filling the usual screen of 
about 25 milliseconds).



  Increase speed of RAM will be manifested as, when necessary 
transfer blocks of information and when you need a conclusion 
similar or common blocks. 

  For static images accelerator required
only to increase the convenience of the output for conversions, 
to display character information. 

  For dynamic images accelerator will
the ability to quickly change the contents of the screen when 
the screen contains a set of identical blocks.



  How else to increase the dynamics of the image, if
accelerator is not enough?

  The simplest solution - switch to more
low level of graphics. In some places the picture may not have 
full raznotsvetiya. In this case, you can transcode mode 
display for these places. For example, replace the usual high 
graphics Spectrum-ovsky. Output mode can be changed at runtime. 
For example, if you move a handsome colored sprite can be 
customized under his regime in the right places the screen. If 
this sprite is displayed by using the accelerator, the high 
dynamics you will be provided. 



  From the Editor.

  Currently, the scheme of graphics accelerators
amended and therefore this article does not
given its parameters. But osnavnaya idea how
seems to us, expressed clearly enough - the new
Spectrum-compatible computer would have
fast graphics.