PICO Picture Compression Demo

Compresses a full TS2068 screen into a smaller block of code and decodes it back again. You pick the address where the coded picture will go (55200 is suggested), then load a SCREEN$, either the next one on the tape or one by name. A 185-byte machine code routine at 55000 encodes the screen and the program reports the coded length next to the size of a normal screen. A keypress clears the display and a second routine at 55130 rebuilds the picture from the coded copy, after which the coded block can be saved as CODE.

A sample screen follows the program on the tape: the "Print a Page" screen from Pixel Print Professional. In testing it compressed to 4771 bytes.

Program Analysis

Program Structure

• Line 400 (the auto-run line): CLEAR 54999, load the pico.C1 machine code, then go to line 30.
• Lines 30–78: ask for the destination address v, store it as a 16-bit value in 23728/23729, and load the screen.
• Lines 100–140: call the encoder at 55000, then read the coded length back from 23728/23729 and print it.
• Lines 150–210: store the address again, wait for a key, and call the decoder at 55130.
• Lines 220–260: optionally SAVE n$ CODE v,x.
• Line 300: saves the BASIC with LINE 400.

The Encoding

The encoder walks the display file and attributes from 16384. Non-zero bytes are copied to the output unchanged. A zero byte is written once and followed by a two-byte count of the zeros in that run, so large blank areas shrink to three bytes each. Runs of any other value are not compressed, so the saving depends on how much of the picture is empty. The decoder reverses the process, copying literal bytes back to the screen and expanding each zero-and-count entry. BASIC and the machine code pass values through the two bytes at 23728/23729: the destination address on the way in and the coded length on the way out.

Notable Techniques

• The two bytes at 23728/23729 carry the parameter and the result between BASIC and the machine code, so no USR return value is needed.
• POKE 23728,v-256*INT (v/256): POKE 23729,INT (v/256) splits the address into low and high bytes.
• The auto-run line comes after the main program and uses GO TO 30 to start it, so CLEAR and the code load happen only once.

Potential Issues

• Line 140 gives the normal screen length as 6192 bytes. A full screen with attributes is 6912 bytes.
• Line 230 loops back to itself instead of to the INPUT at line 220, so any answer other than y or n hangs the program until BREAK.

This program comes from a Larken disk in the TS User Group collection. For tape, the LKDOS prefixes were removed, the 16-bit LKDOS POKE of the address was replaced with two ordinary POKEs, and the disk-load option now loads a screen by name from tape. It was tested in ZEsarUX.