.Page
;++
;
; NRead16
;
; NRead16 assumes that the disk is spinning at the proper speed
; and the head is located at the proper Side/Track and that the header
; for the sector desired has already been found. First it will read in
; the beginning data marks, then the data, and finally the trailing data
; bytes. It will abort if the proper starting or ending sequence of
; data bytes is not found.
;
; REGISTERS
; IN
; All = Any value
; OUT
; All = Destroyed
;
;--
.Org 1800
.Page
Read16 .Equ *
Jsr SetRMode ; Setup PAL on Sony to read mode
Jsr RDSynTop ; Read the sync nibbles
Bcc $03 ; If bitslip marks found then carry = 0
Rts
$03 Ldy #0F4 ; 2 15 First time only 12 bytes
RdBF01 .Equ *
Inc InxPtrH ; 5 28 Point to next page
RdBF02 .Equ *
Ldx Q7L ; 4
Bpl RdBF02 ; 2,3
Lda DNibl,x ; 4 10 Map from 8 bits ==> 6 bits
Asl A ; 2 12
Asl A ; 2 14
Tax ; 2 16 Save temporarily
Asl A ; 2 18
Asl A ; 2 20
Sta Temp1 ; 3 23 >BBCC0000<
Txa ; 2 25
And #0C0 ; 2 27 >AA000000<
$10 Ldx Q7l ; 4 Read low bits of byte A
Bpl $10 ; 2,3
Ora DNibl,x ; 4 10 Combine low and high bits
Sta @InxPtrL,y ; 6 16
Lda Temp1 ; 3 19 >BBCC0000<
Iny ; 2 21
Bne $20 ; 2,3 23/24
Inc InxPtrH ; 5 28 Point to next page
$20 Ldx Q7L ; 4 Read low bits of byte B
Bpl $20 ; 2,3
And #0C0 ; 2 8 >BB000000<
Ora DNibl,x ; 4 12 Add low & high bits
Sta @InxPtr,y ; 6 18
Iny ; 2 20
Beq RdCSM01 ; 2/3 22/23
Lda Temp1 ; 3 24 >BBCC0000<
Asl A ; 2 26
Asl A ; 2 28
$30 Ldx Q7L ; 4 Read low bits of byte C
Bpl $30 ; 2,3
And #0C0 ; 2 8 >CC000000<
Ora @DNibl,x ; 4 12 Add low and high bits
Sta @InxPtr,y ; 6 18
Iny ; 2 20
Beq RdBF01 ; 2/3 22/23 Inc to next page if = 0
Bne RdBF02 ; 3 25 Branch always taken
.Page
RdCSM01 .Equ * ; Read 4 bytes of checksum
Ldx Q7L ; 4
Bpl RdSCM01 ; 2,3
Lda DNibl,x ; 4 10 Map from 8 bits ==> 6 bits
Asl A ; 2 12
Asl A ; 2 14
Tay ; 2 16 >AABBCC00<
And #0C0 ; 2 18 >AA000000<
$10 Ldx Q7L ; 4 Read low bits of byte A
Bpl $10 ; 2,3
Ora Dnibl,x ; 4 10 Add low and high bits
Sta Cksum1 ; 3 13
Tya ; 2 15
Asl A ; 2 17
Asl A ; 2 19
Tay ; 2 21 >BBCC0000<
And #0C0 ; 2 23 >BB000000<
$20 Ldx Q7L ; 4 Read low bits of byte B
Bpl $20 ; 2,3
Ora DNibl,x ; 4 10 Add high and low bits
Sta Cksum2 ; 3 13
Tya ; 5 15
Asl A ; 2 17
Asl A ; 2 19 >CC000000<
$30 Ldx Q7L ; 4 Read low bits of byte C
Bpl $30 ; 2,3
Ora DNibl,x ; 4 10 Combine low and high bits
Sta Cksum3 ; 3 13
Rd9 Lda Q7L ; 4 Check bitslip mark 1
Bpl Rd9 ; 2,3
Cmp DatMk4 ; 3
Bne BSErr ; 2,3
Rd10 Lda Q7L ; 4 Check bitslip mark 2
Bpl Rd10 ; 2,3
Cmp DatMk5 ; 3
Bne BSErr ; 2,3
Clc
RdExit Lda Q6H ; Sense mode
RdWaste Rts
.Page
BSErr Inc BSCnt ; Trailing bitslip mark & error counter
Sec
Bcs RdExit ; Return to Sense mode & exit
CSErr Inc CsErr ; Checksum error counter
RdErr Sec
Bcs RdExit
RdErr2 Inc StSlp ; Starting bitslip error count
Sec
Bcs RdExit
RdSynTop .Equ * ; Starting to read starting bitslip marks
Lda Q6L ; Subroutine to read starting bitslip marks
Ldx #000 ; Switch from sense to read mode
Ldy #NiblRetr ; No. of retries allowed for bitslip read
RSync Dey
Beq RdErr2
RS1 Lda Q7L ; 4
Bpl RS1 ; 2,3 6
RSync1 Cmp DatMk1 ; 3 9 Data mark 1
Bne RSync ; 2,3 11,12
Stx InxptrL ; 3 14
RS2 Lda Q7L ; 4
Bpl RS2 ; 2,3 6
Cmp DatMk2 ; 3 9 Data mark 2
Bne RSync1 ; 2,3 11,12
Stx InxptrH ; 3 14
RS3 Lda Q7L ; 4
Bpl RS3 ; 2,3 6
Cmp DatMk3 ; 3 9 Data mark 3
Bne RSync1 ; 2,3 11,12
Clc ; 2 13
RS4 Lda Q7l ; 4 Read sector number ( unchecked )
Bpl RS4 ; 2,3 6
Rts ; Back to caller
