Z80:Edit Buffers

An edit buffer is a place where user input is stored. It can be added to, have something inserted in the middle, have something deleted, and a whole lot more. An example of a text edit buffer would be Notepad. The data being manipulated may be very different, but the general idea is usually close to this.

Fixed-Length Buffers

Fixed length buffers are area in memory defined to hold the user input. Commonly, they are used for high score menus where users can input there name. They are fixed length because they have a defined beginning and ending point, and generally are continuous (no gaps between parts of the buffer).

Setting it up

To set up a buffer, you need to define the beginning or end. Whether you do this in code or just in your head, it doesn't really matter.

   ;start of our buffer is appbackupscreen, end is appbackupscreen+20

   .define bufStart appbackupscreen
   .define bufEnd appbackupscreen+20

   bufStart:
   .db 0,0,0,0,0,0,0,0,0,0
   .db 0,0,0,0,0,0,0,0,0,0

We'll also need a buffer pointer. This is where the user input is being stored.

   .define bpoint appbackupscreen+21

   bpoint:
   .db 0

   ld b,0

The Code

Buffer Table

So now that we have a block of memory defined for our buffer, we need someway for user input to be stored. Since most buffers hold text inputted from users, we'll create code that allows users to press buttons corresponding with the letter. To do this, we'll use a table of values (for size and speed).

(copied parts from buffer tables)

   Table:
    .db 'A'        ;9A        kCapA
    .db 'B'        ;9B        kCapB
    .db 'C'        ;9C        kCapC
    .db 'D'        ;9D        kCapD
    .db 'E'        ;9E        kCapE
    .db 'F'        ;9F        kCapF
   
    .db 'G'        ;A0        kCapG
    .db 'H'        ;A1        kCapH
    .db 'I'        ;A2        kCapI
    .db 'J'        ;A3        kCapJ
    .db 'K'        ;A4        kCapK
    .db 'L'        ;A5        kCapL
    .db 'M'        ;A6        kCapM
    .db 'N'        ;A7        kCapN
    .db 'O'        ;A8        kCapO
    .db 'P'        ;A9        kCapP
    .db 'Q'        ;AA        kCapQ
    .db 'R'        ;AB        kCapR
    .db 'S'        ;AC        kCapS
    .db 'T'        ;AD        kCapT
    .db 'U'        ;AE        kCapU
    .db 'V'        ;AF        kCapV
   
    .db 'W'        ;B0        kCapW
    .db 'X'        ;B1        kCapX
    .db 'Y'        ;B2        kCapY
    .db 'Z'        ;B3        kCapZ
    .db 'X'        ;B4        kVarX

Key presses and bound checking

That table works well with the Getkey ROM call. We'll also need a way to check for invalid keypresses. Since all the values greater than $B4 are invalid, and all the values less than $9A are also invalid, and everything else is valid, we'll use these limits as our check. We also need a way to check for special keys (keys that when pressed perform a special action, such as pressing [ENTER] to finish editing, or [CLEAR] to empty the buffer, etc.), and a limit test so we don't overwrite the buffer's bounds.

   Keyloop:
    bcall(_GetKey)
   
    cp kEnter
    jr z,editDone
   
    ld c,a
    ld a,b
    cp 20
    jr nc,Keyloop
    ld a,c
   
    cp $B5
    jr nc,Keyloop
    cp $9A
    jr c,keyloop

Data conversion and saving

And now for a way to translate key pressed into the data we want.

   ld hl,table
   ld e,a
   ld d,0
   add hl,de
   ld a,(hl)

To finish, we'll store this value into the buffer, and advance the buffer counter, and repeat.

   ld hl,bufStart
   ld e,b
   ld d,0
   add hl,de
   ld (hl),a
   jr keyloop

Working fixed-length buffer

And the full code:

   start:
    ld b,0						;buffer pointer
   Keyloop:
    bcall(_GetKey)
   
    cp kEnter						;special key enter
    jr z,editDone
   
    ld c,a						;check to see if buffer is full
    ld a,b
    cp 20
    jr nc,Keyloop
    ld a,c
   
    cp $B5						;check for valid input
    jr nc,Keyloop
    cp $9A
    jr c,keyloop
   
    ld hl,table						;get character
    ld e,a
    ld d,0
    add hl,de
    ld a,(hl)
   
    ld hl,bufStart					;save
    ld e,b
    ld d,0
    add hl,de
    ld (hl),a
   
    jr keyloop
   
   editDone:
    ret						;for now, just quit
   
   bufStart;
   .db 0,0,0,0,0,0,0,0,0,0
   .db 0,0,0,0,0,0,0,0,0,0
   Table:
    .db 'A'        ;9A        kCapA
    .db 'B'        ;9B        kCapB
    .db 'C'        ;9C        kCapC
    .db 'D'        ;9D        kCapD
    .db 'E'        ;9E        kCapE
    .db 'F'        ;9F        kCapF
   
    .db 'G'        ;A0        kCapG
    .db 'H'        ;A1        kCapH
    .db 'I'        ;A2        kCapI
    .db 'J'        ;A3        kCapJ
    .db 'K'        ;A4        kCapK
    .db 'L'        ;A5        kCapL
    .db 'M'        ;A6        kCapM
    .db 'N'        ;A7        kCapN
    .db 'O'        ;A8        kCapO
    .db 'P'        ;A9        kCapP
    .db 'Q'        ;AA        kCapQ
    .db 'R'        ;AB        kCapR
    .db 'S'        ;AC        kCapS
    .db 'T'        ;AD        kCapT
    .db 'U'        ;AE        kCapU
    .db 'V'        ;AF        kCapV
   
    .db 'W'        ;B0        kCapW
    .db 'X'        ;B1        kCapX
    .db 'Y'        ;B2        kCapY
    .db 'Z'        ;B3        kCapZ
    .db 'X'        ;B4        kVarX