Sunday, July 17, 2022

Vic-20 HI-RES in progress - Memory Layout

Vic-20 Graphics Memory Layout

The Vic-20 doesn't have a native graphics mode.  But it can use a character set from onboard RAM, and double the size of character cells from 8x8 to 8x16 pixels.


A solution puts video memory at address 4096 ($1000), and character RAM at ($1000).  Notice they are the same?  Yes, there's only 5K onboard RAM in addition to color RAM.   The first 1K starts at address 0 ($0000), and the other 4K starts at address 4096 ($1000).   Regardless of any other RAM present in the system (can add 35K), only that latter 4K is accessible as video/character RAM to the video chip.  (Even if the earlier 1K is available, there's too many conflicts with BASIC and KERNAL use).

So bitmapped characters (graphics) and video RAM (characters on the screen) have to share the same section of memory.  The way bytes work, at most 256 characters could be displayed on the screen, but that would require a full 4K of RAM for their bitmaps (16*256), so something has to give.   By using only 240 characters on the screen, that reserves 16*16 = 256 bytes for video memory, and the remaining memory can be used for graphics.  For the bitmap to skip over screen memory and avoiding any overlap, the characters used on the screen are indexed starting at 16 ($10).

$1000-$10EF screen (240 bytes), values $10-$FF (16 to 255)
$10F0-$10FF unused (16 bytes)
$1100-$1FFF bitmap (3840 bytes)

BASIC RAM can be moved to either 3K expansion RAM ($400-$FFF) or 8K or greater expansion RAM ($2000 and later).

One layout that is possible is 20 characters wide by 12 characters tall = 240 characters = 160x192 resolution.

The easiest layout of characters is to store them consecutively in memory as shown above.   Character layout for the screen is left to right, then wraps to the next row.   This scheme matches color memory layout as well.   An additional advantage is that time sensitive updates to graphics can be more aligned with the raster line.

Another layout of characters is to line them up vertically as shown below.  This has the advantage that memory is contiguous vertically until advancing to the next column of 8 pixels.   This allows for optimizations in managing graphics memory at the disadvantage of working vertically instead of horizontally, thus fighting the raster line.  But color memory is still horizontally oriented, so the address calculation is different, the same as the earlier layout.

 
As the Vic-20 allows for configuring the number of character columns, character rows, and adjusting top and left margins, a variety of resolutions are possible.

Some notable resolutions include

160x192, 192x160, 200x144, 144x208, 160x160

The last is mentioned as it is the resolution used by the Super Expander cartridge.  But note it only requires 200 bytes of screen memory, so using that resolution leaves 696 bytes of onboard memory unused for graphics/screen that could be available for another use, maybe 506 bytes is still text screen memory. 

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2 comments:

  1. The Vic20 is the only platform from that era that I know of that has variable screen size. It's just a shame that the current screen dimensions were never used for the CBM kernal routines. Instead they used the fixed constants in ROM.

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  2. Hawk: You can use my "Maxiedit"-tool to use multiple screen-resolutions for the screen-editor:

    http://sleepingelephant.com/ipw-web/bulletin/bb/viewtopic.php?f=2&p=92294

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