Screen 2 Show

Create animated displays for MSX screen 2 images.

Project details / Version that Compresses image (old)
The Screen 2 Show project involves both MSX and PC computers. At the MSX side, aims at developing display algorithms in Assembly. At the PC side, aims at developing tools to help on adding the display code to images converted to MSX.
Currently, this project works only with MSX 1 screen 2 format. Thus, it is extensible to other screen formats.

How the program works

        File                      RAM                  RAM                  VRAM

 10 SCREEN 2                9000 | MMMPPP |      4000 | PPPPPP |      0000 | PPPPPP |
 20 BLOAD"TELA.SCR",R  ->   .... | CCDD   |  ->  .... |        |  ->  .... |        |
 30 GOTO 30                 FFFF |        |      5800 | CCCCCC |      2000 | CCCCCC |

The first action is to load the program into the RAM, starting at the address &H9000. It is divided into:

The main program (marked as a green "M")
The compacted image (marked as a blue "P" for pattern image and a red "C" for color image)
The display program (marked as cyan "D")

The main program extract the compressed image into the RAM region starting at address &H4000. The extraction could be permormed directly to the VRAM, but it should increase considerably the complexity of the display algorithms.
Once extracted the image, the main program calls the display program. This is responsible for copying data from RAM to VRAM. The resulting animation is due to the way the data is copied to the VRAM.

File layout

Part	MSX Address
Header	9000
Main code	900B
RLE compressed picture	9061
Display code	xxxx

xxxx → Undefined address.

Header description

MSX Address	Value	Description
-	FE	MSX Binary file ID
-	00	Program beggining
-	90	Program beggining (9000)
-	xx	End of File
-	xx	End of file
-	0B	Exec beggining
-	90	Exec beggining (900B)
9000	xx	Size of compressed image (pat)
9001	xx	Size of compressed image (pat)
9002	xx	Size of compressed image (col)
9003	xx	Size of compressed image (col)
9004	xx	Type of display: 00 - Direct 01 - Top to bottom 02 - Left to right 03 - Diagonal 04 - Snail in 05 - Frame
9005	4D	"M"
9006	61	"a"
9007	72	"r"
9008	4D	"M"
9009	53	"S"
900A	58	"X"

- → Not used address.
xx → Undefined value.

Image compress method

Image compress method used is simply RLE (Run Length) which works as follows:

Suppose an string "AAABCCCDDD"

Each sequence of the same data is counted, resulting on a pair of byte:

- total (max=255)
- value of repeated byte

At the end of the file, a byte valued as 0 is introduced to indicate the end of data.
According to that, the original string becomes to: 3A 1B 2C 3D 0

On the MSX 1 screen 2 pictures, first we compact pattern image area from 0000H to 1800H (VRAM). Then, we do the same to color area from 2000H to 3800H.

Main code

   10 		ORG  &HC000
   20 		CALL &H72		; INIGRP (Screen 2)
   30 		IN   A,(&HA8)		; *
   40 		CALL PG2		; * Adjust slots to RAM RAM RAM xxx
   50 		OUT  (&HA8),A		; *
   60 		LD   A,(&HFFFF)		; %
   70 		CPL 			; % 
   80 		LD   (&H8FFF),A		; % Adjust page 2 subslot to RAM
   90 		CALL PG2		; %
  100 		LD   (&HFFFF),A		; %
  110 		LD   HL,&H4000		; Start of dest. RAM to uncompress (pattern)
  120 		LD   DE,&H9061		; Start of compressed data
  130 		CALL LE1		; Call uncompress routine
  140 		INC  DE			; Increments compress data pointer (color)
  150 		CALL LE1		; Call uncompress routine
  160 		CALL TELA		; Call routine to copy RAM to VRAM
  170 		IN   A,(&HA8)		; *
  180 		LD   B,A		; *
  190 		LD   A,&HF0		; * Returns original configurations
  200 		AND  A			; *
  210 		OUT  (&HA8),A		; *
  220 		LD   A,(&H8FFF)		; % Returns original configurations
  230 		LD   (&HFFFF),A		; %
  240 		RET 			; END
  250 PG2:	LD   B,A		; *
  260 		LD   A,&B11110011	; *
  270 		AND  B			; *
  280 		LD   B,A		; *
  290 		LD   A,&B00110000	; * Copy routine: xx AA xx xx -> xx AA AA xx
  300 		AND  B			; *
  310 		SRA  A			; *
  320 		SRA  A			; *
  330 		ADD  A,B		; *
  340 		RET 			; *
  350 LE1:	LD   A,(DE)		; %
  360 		OR   0			; %
  370 		JP   Z,FIM		; %
  380 		LD   B,A		; %
  390 		INC  DE			; % Uncompresses data. Found 0 items, ends.
  400 		LD   A,(DE)		; %
  410 		INC  DE			; %
  420 LI1:	LD   (HL),A		; %
  430 		INC  HL			; %
  440 		DJNZ LI1		; %
  450 		JP   LE1		; %
  460 FIM:	RET 			; %

The main code must be compiled to the memory, starting at the RAM address &H900B.

Display codes

  470 TELA:	LD   BC,&H1800		; $
  480 		LD   DE,0		; $
  490 		LD   HL,&H4000		; $
  500 		CALL &H5C		; $ Particular routine to display screen
  510 		LD   BC,&H1800		; $ In this case, copies direct
  520 		LD   DE,&H2000		; $
  530 		LD   HL,&H5800		; $
  540 		CALL &H5C		; $
  550 		RET 			; $

The display code must be compiled to the memory, starting at the RAM address imediately after the compressed image. In this case, the position depends on the size of the compressed data.
The main program should be fixed on the line 160, changing "TELA" to the display code starting address.

Display codes
Support: Screen 2 Achitecture