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. 

Links

Vic-20 open source hi-res support (github)

Built D64 disk image for Vic-20 is here and see Usage in README.md

Extending C64 BASIC Part Three - Poke multiple bytes

This example accepts one address value, followed by multiple byte values appended to the SYS command on Commodore 64.  

This could be useful for POKEing to screen memory, sprite data, programmable character data, consecutive I/O registers, etc. efficiently in a single statement.

Links:

ACME cross assembler (optional)

poke_bytes.d64

open source (github)

Extending C64 BASIC Part Two - Parse 16-bit unsigned integer

This example parses a 16-bit unsigned integer appended to the SYS command on Commodore 64.  

The number is added to the jiffy clock as the alarm time, and a busy loop is entered waiting for the jiffy clock to advance to the alarm time.

Links:
ACME cross assembler (optional)
delay_jiffies.d64
open source (github)

Slides for C64

Forget PowerPoint or Google Slides or Apple Keynote.   Here is a newer solution for the Commodore 64.   Simply use DATA statements in BASIC to define the layout of your slides.  What could be easier than that?

Links:

• Source on github
• README.md including instructions, references, and more details
• Slides.D64 disk image for Commodore 64

easymenu64 - directory and program launcher utility for C64

 

I originally wrote this somewhere between 1985 and 1987.  I submitted the program and an article to Compute! publications but was rejected, probably not unique or attractive enough of a submission.  Well, here it is for the public to see approximately 35 years later.  Not sure if I wrote this originally in assembler, or just machine code.  I suspect I used machine code for this one.  I have done a fair job now disassembling and crafting into assembler (using ACME cross assembler) for a more modern experience.

My typical use was to either see what was on the disk, or as a game launcher.  Imagine the possibility of many games packed on a 1581 disk.  It even supports launching cartridge images.

I have used it with 1541/1571/1581 but not sure if it works with fully populated large drives.  Seems to have trouble with some non-standard drives.  I plan to review that in the future.  (Update: some fixes have been applied to the original code starting with tagged version 1.1)

Keyboard and joystick(#2) controls are documented with the source README.

Links:
    open source
    easymenu.d64 (disk image)

Extending C64 BASIC Part One - Parse String

This example parses a string appended to the SYS command on Commodore 64

Links: 

ACME cross assembler (optional)

reverse.d64 (updated 2022-05-29 to include source and example startup)

open source (github)

GO 128 enhancement for Commodore

 

One wonderful feature of the Commodore 128 was its compatibility with the Commodore 64.  It was an evolution of the Commodore 64 and still supported all the software and hardware from before.  For me it was a no-brainer to buy.  80 columns, plus new features, plus CP/M.   Adding a mouse, GEOS 128, REU 512K, and 3.5" floppy, and I thought it was the best place to be this side of heaven.

There were multiple ways to go into Commodore 64 mode.

1. Start with a Commodore 64 cartridge
2. Boot/reset with the Commodore key held
3. Commodore 128 boot disk that switched to Commodore 64 mode
4. Switch MMU to Commodore 64 mode
5. GO 64 command

But the only way to get back into Commodore 128 BASIC 7.0 was to reset the system.

Fast forward a few decades or so, and now we can add GO commands to the other BASIC systems to switch between them.  Links: cross-platform source, Windows binary release.

How does it work?  This is emulation.  Sorry purists.   The Commodore 128 MMU was built so there is no going back out of C64 mode.  This was by design to keep 64 mode on the 128 very close to 100% compatibility.  But emulation?  Anything goes.  The GO command is hooked, the argument is checked for a number, and if the number is right, it goes to that Commodore model by restarting the emulation for that system.  And thus, when running the BASIC for one system, can quickly switch to another other system (or even reset the system) using the GO command.  It even prompts for amount of RAM to allocate (except 128 is fixed at 128).

PET 2001 2K to 64K supported 

Vic-20 5K to 40K supported in hardware supported increments, not all available to BASIC

64 RAM 3K to 64K supported not all available to BASIC

Commodore 16 and Plus/4 support 16K, 32K, or 64K

Commodore 128 fixed at 128K (KERNAL does not do RAM test) 

Limitations?  Oh, sorry.  This emulator is text only.  No full screen editor.  Doh!   No screen pokes either (well not displayed).  [Update 6/2021:] PETSCII and commodore graphics characters supported with third party font.  Does support console history editor on Windows.   Linux and MAC builds (you can build from my open source project) provide only simple line editing capability.   Does provide accurate 6502/6510/8502 emulation, RAM banking, etc. according to the platform.   So pop into the MONITOR on C128 or C16, Plus/4, or SUPERMON on other platforms to write and edit machine code.   And because using console window, the text display size is not as limited, 88 columns for Vic-20 and 160 columns for C128.

Save and Load from .PRG is supported as well (except PET at this time, some other limitations).  Disk device is ignored, so defaults to local file system.  Even use absolute and relative paths.

So for simple programs, and just playing around in BASIC, it's not too bad.   Text background and foreground color for Vic-20 is enabled because that color palate is more compatible with the Windows console default colors.  Reverse text should work on all systems.  Home, clear, cursor positioning may work in programs only, not for editing.

Now prompts for RAM size going to 64

Going to Vic-20 from 64

Going to PET 2001 from Vic-20

Going to Plus/4 from Vic-20

Going to Commodore 16 from Plus/4

Going to 128 from Commodore 16

Resizable text console - 132x50 text screen on Commodore 128

Bil Herd, Ted/C128 hardware design project lead (left) and Dave Van Wagner, geek (right)

Links: 

• cross-platform source 
• Windows binary release