MESS OUT

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MESS_OUT ($C004CE): Generic VRAM output

Mess out.png

MESS_OUT executes the command buffer in the System ROM's work RAM that starts at BIOS_MESS_BUFFER (constant $10FF00) and goes up to BIOS_MESS_POINT (longword variable $10FDBE). This can be used to display text messages, boxes... on the fix layer (is it used for sprites too ?).

MESS_OUT is automatically called in the SYSTEM_INT1 call. BIOS_MESS_BUSY ($10FDC2.b) can be used to prevent this (when setting up the command list, for example).

The command buffer can be made of commands directly, or pointers to lists in ROM or user RAM. To put commands directly into the buffer, a null pointer ($00.l) needs to be placed before the command list.

Commands are always words.

Commands

Command 0: End of command list

1 word: $0000

Command 1: Set data format

2 words:

Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Def 0 Byte/WordEnd code/Data size$01

This command specifies if the data will be in bytes or words, and if the size is defined or if an end code has to be reached.

If data is in bytes (bit 9 = 0)
The next word's upper byte will be the data's constant upper byte (since VRAM can only be written in words).
If the data is limited by an end code (bit 8 = 0), the next word's lower byte will be the end code's value.
If the data size is defined (bit 8 = 1), the next word's lower byte will be the size (so max length is $FF).
If data is in words (bit 9 = 1)
If the data is limited by an end code (bit 8 = 0), the next word will be the end code's value.
If the data size is defined (bit 8 = 1), the next word will be the size (so max length is $FFFF).

Examples:

dc.w  $0001,$15FF  ;Data will be read in bytes, upper byte will always be $15, end code is $FF.
dc.w  $0101,$1520  ;Data will be read in bytes, upper byte will always be $15, data length is $20.
dc.w  $0201,$8000  ;Data will be read in words, end code is $8000.
dc.w  $0301,$2044  ;Data will be read in words, data length is $2044.

The format stays the same until command 1 is used again.

Command 2: Set auto-increment

  • 1 word
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Def Auto-inc value $02

Sets the auto-increment value (written to VRAM_MOD ($3C0004)). Is the value sign-extended or not ?

dc.w  $2002  ;Set auto-inc to $20

Command 3: Set VRAM address

  • 2 words

Sets the VRAM address written to VRAM_ADDR ($3C0000)) to the next word's value.

dc.w  $0003,$7201  ;Set VRAM address to $7201

Command 4: Set data address

  • 3 words

Sets the output data address to the next longword. Actual writes to VRAM are done with this command.

dc.w  $0004
dc.l  MESSAGE

Command 5: Add to current VRAM address

  • 2 words

Add the next word to the current VRAM address. (This is different from command 2). Useful to skip lines or spaces.

dc.w  $0005,$001F  ;Add $1F to the current VRAM address

Command 6: Resume data output

  • 1 word

Resume data output (instead of using command 4 again, keep going from the last address).

Command 7: Directly define output data

  • at least 2 words

Directly define the data (instead of using command 4 to point to it). Don't forget the padding byte if the data ends on an odd address.

dc.w  $0007
dc.b  $30,$31,$34,$45,$FF
dc.b  $00    ;Pad byte

Command 8: Write text with 8x16 font

  • at least 2 words
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Def Upper data byte (fontset number) $08

Writes with the 8*16 pixels fix font. The commands upper byte will be the data's constant upper byte. The next bytes are data, and the end code is $FF.

The auto-inc is automaticaly set to $20.

dc.w  $0108    ;Tileset 1
dc.b  $44,$51  ;Data
dc.b  $FF      ;End code
dc.b  $00      ;Pad byte

Command 9: Write Japanese text

Same as command 8 but for Japanese characters.

Command A: Call sub list

  • 3 words

Call sub command list. The next longword is a pointer to another command list. Return has to be made using command B.

dc.w  $000A
dc.l  SUBLIST

The call stack seems to be 5 addresses deep. Be sure to avoid nesting too many list calls.

Command B: Return from sub list

  • 1 word

Return to calling command list.

Command C: Repeated data output

  • 2 words

Repeat output. The upper byte of the command is the number of times to output the next word.

dc.w  $090C,$0147  ;Outputs $0147 9 times

Command D: Repeat and increment output

  • 2 words

Repeat and increment output. The upper byte of the command is the number of times to output the next word. The lower byte of the data is incremented each time.

dc.w  $040D,$42FE  ;Outputs $42FE,$42FF,$4200,$4201

Example code and command lists

(Slightly modified code from SNK's doc, output gives the above screencap.)

 bset.b  #0,BIOS_MESS_BUSY
 movea.l BIOS_MESS_POINT,a0  ;Get current pointer in buffer
 move.l  #0,(a0)+       ;Direct commands
 move.w  #3,(a0)+       ;Set VRAM address to $7318
 move.w  #$7318,(a0)+
 move.w  #$0301,(a0)+   ;Words, size = 1
 move.w  #$0001,(a0)+
 move.w  #$0007,(a0)+   ;Output data
 move.w  $100000,(a0)+
 move.w  #0,(a0)+       ;End of this list
 move.l  #MESSAGE1,(a0)+ ;Pointer to list in ROM
 move.l  a0,BIOS_MESS_POINT  ;Update pointer
 bclr.b  #0,BIOS_MESS_BUSY   ;Ready to go
 rts

MESSAGE1:
  dc.w  $0001    ;Bytes, upper = $00, end code = $FF
  dc.w  $00FF
  dc.w  $2002    ;Auto-inc = $20
  dc.w  $0003    ;VRAM address = $7024
  dc.w  $7024
  dc.w  $0004    ;Write "MESSAGE1"
  dc.l  MS1

  dc.w  $0005    ;Next line
  dc.w  $0001
  dc.w  $0006    ;Resume, write "MESSAGE2"
  dc.w  $0005    ;Next line
  dc.w  $0001

  dc.w  $0007    ;Direct data output
  dc.b  "MESSAGE3"
  dc.w  $FF00    ;End code and pad byte

  dc.w  $0005    ;Next line
  dc.w  $0001

  dc.w  $0108    ;8x16 write, fontset 1
  dc.b  "ABCDE",$FF
  dc.w  $0005    ;Next line
  dc.w  $0001
  dc.w  $0109    ;8x16 write, fontset 1
  dc.b  0,1,2,3,4,$FF
  dc.w  $0005    ;Next 2 lines
  dc.w  $0002
  dc.w  $000A    ;Sub list calls
  dc.l  SUB_MESS
  dc.w  $000A
  dc.l  SUB_MESS
  dc.w  $000A
  dc.l  SUB_MESS
  dc.w  $0000

MS1:
  dc.b "MESSAGE 1",$FF
  dc.b "MESSAGE 2",$FF

SUB_MESS:
  dc.w  $090C    ;Outputs $0022 9 times
  dc.w  $0022
  dc.w  $070D    ;Outputs $0042 7 times with inc
  dc.w  $0042
  dc.w  $0005    ;Next line
  dc.w  $0001
  dc.w  $000B    ;Return