; Ultim809 keyboard handling code, 1/26/11
; yes I probably could use direct addressing for almost all of this,
; but I don't want to steal the direct page register from a user program;
; I'm not sure I want to prevent users from using the DPR at this point
; but it would make several ROM routines faster

; The scancode buffer is 16 bytes and is aligned on a 16-byte boundary
; Even though the head and tail pointers are 2 bytes, we can get away with
; manipulating only the low bytes.
KBD_SAVE    .equ    0x58    ;used to save registers in the interrupt handler
KBD_BITSLEFT    .equ    0x5A    ;number of bits left to read
KBD_SCANCODE    .equ    0x5B    ;scancode being received
KBD_HEADPTR .equ    0x5C    ;pointer to the head of the buffer
KBD_HEADPTR_L   .equ    0x5D
KBD_TAILPTR .equ    0x5E    ;pointer to the tail of the buffer
KBD_TAILPTR_L   .equ    0x5F
KBD_BUFSTART    .equ    0x60    ;start of the buffer
KBD_BUFEND  .equ    0x6F    ;end of the buffer
KBD_BUFMASK .equ    0b11101111


;;; keyboard interrupt handler (FIRQ)
;;; typical PS/2 clock is 15 kHz (66.67 us period) so this should be fine
KBD_HANDLER:    tst PIA_PRA     ;clear interrupt source
        dec KBD_BITSLEFT
        beq kbh_done    ;parity bit, ignore
        bmi kbh_stopbit ;last bit, store scancode in buffer
        sta KBD_SAVE    ;save A register
        lda PIA_PRA     ;read data bit
        lsla
        ror KBD_SCANCODE    ;shift into scancode
        lda KBD_SAVE    ;restore A register
kbh_done:   rti
kbh_stopbit:    sta KBD_SAVE    ;save A register
        lda KBD_SCANCODE    ;store scancode
        sta [KBD_HEADPTR]
        lda KBD_HEADPTR_L   ;advance head pointer
        inca
        anda    #KBD_BUFMASK    ;force wraparound
        sta KBD_HEADPTR_L
        lda #10     ;reset bit counter
        sta KBD_BITSLEFT
        lda KBD_SAVE    ;restore A register
        rti


;;; fetch a scancode from the buffer
;;; arguments:  none
;;; returns:    scancode in B or 0 if buffer is empty
;;; destroys:   A
KBD_GETCODE::   ldb #10
1$:     cmpb    KBD_BITSLEFT    ;wait until a full scancode is processed
        bne 1$
        ldb KBD_HEADPTR_L   ;is the buffer empty?
        cmpb    KBD_TAILPTR_L   ;(i.e. head and tail are the same)
        beq bufempty
        ldb [KBD_TAILPTR]   ;buffer not empty, get code
        lda KBD_TAILPTR_L   ;advance tail pointer
        inca
        anda    #KBD_BUFMASK    ;force wraparound
        sta KBD_TAILPTR_L
        tstb            ;set flags for return
getcodedone:    rts
bufempty:   clrb            ;return 0
        bra getcodedone