83Plus:Hooks:9B88

Synopsis

Name: GetCSC (officially GetKey; this name is discouraged)

Hook Pointer Block Address: 9B88

Hook Enable BCALL: 4F7B

Hook Disable BCALL: 4F7E

Hook Call BCALL: 4F78

Hook Active Flag: 0, (iy + 34h)

This hook is called when the OS scans for keypad activity. It is called once before the key scan begins, and a second time if the scan finds a key pressed. It thus operates at a lower level than the Raw Key hook, which operates on GetKey values.

Using the Hook

These different values, passed in A, determine what the hook should do.

• 1Ah: Before the keypad is scanned
  • Return with Z set and A = scancode to force a key to be "pressed."
  • Return with NZ set and A = 1Ah to scan the keypad normally.
• 1Bh: After the keypad is scanned and a key is found
  • B = scancode found
  • Return with NZ set and A = scancode to be returned.

Comments

Officially named the "GetKeyHook." This name is deprecated and confusing.

Example

The following code will swap the 2nd and Alpha keys:

GetKeyHook:
        .db 83h             ; Required for all hooks
        cp 1Bh              ; which condition?
        ret nz
        ld a,b              ; which key was pressed?
        cp skAlpha          ; was it Alpha?
        jr z,AlphaKey
        cp sk2nd            ; was it 2nd?
        ret nz
        ld a,skAlpha-1      ; change to Alpha
        inc a               ; set NZ
        ret
AlphaKey:
        ld a,sk2nd-1        ; change to 2nd
        inc a               ; set NZ
        ret

Credits and Contributions

• Texas Instruments (thanks for all the confusing equates)