Introduction
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 9ドルA 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 9ドルA
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 9ドルA
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