83Plus:Hooks:9B88

Synopsis

Name: GetCSC (officially GetKey; there is some disagreement as to the appropriateness of both names)

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. (Note, however, that it is only called when the OS is idle in the GetKey routine.) 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 actual 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)