This document tries to cover all of the incompatible SAV file format changes
and counter addition/removals since 2.51s1.

Going back further would be valuable. Contributions from Terryn and ajs.

== MZX 2.51s1 =================================================================

+ Added MOUSEX, MOUSEY, BUTTONS, MBOARDx, MBOARDy, SCROLLEDx, SCROLLEDy,
  PLAYERX, PLAYERY, CURSORSTATE, LOCAL

== MZX 2.51s2 =================================================================

+ Added MOD *

== MZX 2.51s3.2 ===============================================================

+ Added BIMESG

== MZX 2.60 ===================================================================

+ File access added: FREAD, FWRITE, FREAD_OPEN, FWRITE_OPEN, FWRITE_APPEND,
  FREAD_POS, FWRITE_POS, FREAD_PAGE/FWRITE_PAGE
  (these two were removed (in 2.80, IIRC - it wasn't used anyway.))
+ KEY_PRESSED
+ INT, BIT_PLACE, INT2BIN
+ CHAR_X, CHAR_Y, PIXEL
+ CHAR_BYTE, CHAR, BYTE
+ R_PLAYERDIST (removed in 2.68)
+ F2_MENU, ENTER_MENU, HELP_MENU
+ BULLETTYPE, LAVA_WALK
+ COMMANDS
+ : "key#"
+ (read_length and write_length counters are claimed,
   but I don't see them working?)

Ak additions incorporated into 2.60:

+ DATE_DAY, DATE_MONTH, DATE_YEAR
+ TIME_HOURS, TIME_MINUTES, TIME_SECONDS
+ MZX_SPEED
+ CURRENT_COLOR, RED_VALUE, GREEN_VALUE, BLUE_VALUE
+ OVERLAY_X, OVERLAY_Y, OVERLAY_CHAR, OVERLAY_COLOR
+ BOARD_X, BOARD_Y, BOARD_COLOR, BOARD_CHAR
+ ROBOT_ID
+ ABS_VALUE, SQRT_VALUE, VALUE (removed in 2.68 in favor of expressions)

== MZX 2.62 ===================================================================

+ Strings added. 10 total; names limited to $string0 through $string9.
  String comparison possible.
+ INPUT, BOARD_NAME, BOARD_SCAN built-in strings.
  FREAD/FWRITE now have added string writing/reading functionality.
+ MOD_ORDER

== MZX 2.65 ===================================================================

+ FREAD_COUNTER, FWRITE_COUNTER
+ BOARD_W, BOARD_H
+ ROBOT_ID_Name
+ R#.Counter
+ Sprites added; limit set at 64 global: SPRn_REFX, SPRn_REFY, SPRn_HEIGHT,
  SPRn_WIDTH, SPRn_X, SPRn_Y, SPRn_CX, SPRn_CY, SPRn_CWIDTH, SPRn_CHEIGHT,
  SPR_COLLISIONS, SPR_CLISTn, SPRn_STATIC, SPRn_OVERLAID, SPRn_CCHECK,
  SPRn_OFF, SPR_YORDER, SPR_NUM, SPRn_SETVIEW, SPRn_SWAP;
  Sprite and Sprite_colliding objects added.
+ Subroutines added, require setting up the stack with a line like
  (. "#*-1-2-3") etc, special "#return" and "#top" labels.
+ Hex outputs with && added using + and # (ex: &+counter&, &#counter&).
  # adds a 0 to the output if the counter is less than 0x10.
+ Partial chars became loadable Through +xxname.char
  (xx is in hex and is the starting position).

== MZX 2.68 ===================================================================

- Added DIVIDER, MULTIPLIER, C_DIVISIONS
- Added SIN, COS, TAN, ATAN, ASIN, ACOS, SQRT, ABS
- Added SAVE_GAME, LOAD_GAME
- Removed R_PLAYERDIST, ABS_VALUE, CHAR_X, WRAP
- MZMs, Strings, Expressions added

Save file breaks due to Sprites, Math, Input/Output file, see below..

+MZX save file format extensions
+
+These all are stored after the counters in the save file.
+
+Byte(s)
+
+00-1023                                Sprites (16 bytes each * 64)
+1024                           Total sprite count
+1025                           Sprite y_order flag
+1026-1091                      Sprite collision info (66 bytes)
+1092-2115                      Strings (16 bytes each * 64)
+2116-2117                      Math multiplier
+2118-2119                      Math divider
+2120-2121                      Circle divider
+2122-2133                      Input file name
+2134-2137                      Input file position
+2138-2149                      Output file name
+2150-2153                      Output file position

== MZX 2.69 ===================================================================

- Added SMZX_PALETTE, SMZX_MODE, SMZX_{R,G,B}
- Added KEY?, KEY_RELEASE, KEY_CODE (possibly just altered, it's unclear)

Save file breaks due to writing SMZX mode, SMZX mode 2 palette (optionally) and
COMMANDS counter.

== MZX 2.69b ==================================================================

- Added ROBOT_NAME
- Added ridName (aka rid?)

Save format got broken again, but for no good reason I can see?

== MZX 2.69c ==================================================================

- Added FWRITE_MODIFY (?)
- Added SAVE_WORLD
- Added VLAYER_WIDTH, VLAYER_HEIGHT, VCH, VCO
- Overlay mode 2 vlayer modification, probably breaks SAV files

Sprites seemed to get a rewrite in this version. It broke the SAV file format
again.

VLayer stuff got written out (width, height, chars, colors) to SAV.

== MZX 2.70 ===================================================================

- Added load_robot, save_robot, load_bc, save_bc (and N variants)

== MZX 2.80+ ==================================================================

This is a bit more work and we haven't started it yet.