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fndrel.src
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fndrel.src
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.page
.subttl fndrel
;********************************
;*
;* find relative file
;*
;* version 2.5
;*
;* inputs
;* recl - 1byte=lo record #
;* rech - 1byte=hi record #
;* rs - 1byte=record size
;* recptr - 1byte=first byte wanted from record
;*
;* outputs
;* ssnum - 1byte=side sector #
;* ssind - 1byte=index into ss
;* relptr - 1byte=ptr to first byte wanted
;*
;********************************
fndrel jsr mulply ;result=rn*rs+rp
jsr div254 ;divide by 254
lda accum+1 ;save remainder
sta relptr
jsr div120 ;divide by 120
inc relptr
inc relptr
lda result ;save quotient
sta ssnum
lda accum+1 ;save remainder
asl a ;calc index into ss
clc
adc #16 ;skip link table
sta ssind
rts
; multiply
;
; result=recnum*rs+recptr
;
; destroys a,x
mulply jsr zerres ;result=0
sta accum+3 ;a=0
ldx lindx ;get index
lda recl,x
sta accum+1
lda rech,x
sta accum+2
bne mul25 ;adjust for rec #1 &...
lda accum+1 ;...#0 = 1st rec
beq mul50
mul25 lda accum+1
sec
sbc #1
sta accum+1
bcs mul50
dec accum+2
mul50 lda rs,x ;copy recsiz
sta temp
mul100 lsr temp ;do an add ?
bcc mul200 ;no
jsr addres ;result=result+accum+1,2,3
mul200 jsr accx2 ;2*(accum+1,2,3)
lda temp ;done ?
bne mul100 ;no
lda recptr ;add in last bit
clc
adc result
sta result
bcc mul400 ;skip no carry
inc result+1
bne mul400
inc result+2
mul400 rts
; divide
;
; result=quotient ,accum+1=remainder
;
; destroys a,x
div254 lda #254 ;divide by 254
.byte $2c ;skip two bytes
div120 lda #120 ;divide by 120
sta temp ;save divisor
ldx #3 ;swap accum+1,2,3 with
div100 lda accum,x ;result,1,2
pha
lda result-1,x
sta accum,x
pla
sta result-1,x
dex
bne div100
jsr zerres ;result=0
div150 ldx #0
div200 lda accum+1,x ;divide by 256
sta accum,x
inx
cpx #4 ;done ?
bcc div200 ;no
lda #0 ;zero hi byte
sta accum+3
bit temp ;a div120 ?
bmi div300 ;no
asl accum ;only divide by 128
php ;save carry
lsr accum ;normalize
plp ;restore carry
jsr acc200 ;2*(x/256)=x/128
div300 jsr addres ;total a quotient
jsr accx2 ;a=2*a
bit temp ;a div120 ?
bmi div400 ;no
jsr accx4 ;a=4*(2*a)=8*a
div400 lda accum ;add in remainder
clc
adc accum+1
sta accum+1
bcc div500
inc accum+2
bne div500
inc accum+3
div500 lda accum+3 ;test < 256
ora accum+2
bne div150 ;crunch some more
lda accum+1 ;is remainder < divisor
sec
sbc temp
bcc div700 ;yes
inc result ;no - fix result
bne div600
inc result+1
bne div600
inc result+2
div600 sta accum+1 ;new remainder
div700 rts
; zero result
zerres lda #0
sta result
sta result+1
sta result+2
rts
; multiply accum by 4
accx4 jsr accx2
; multiply accum by 2
accx2 clc
acc200 rol accum+1
rol accum+2
rol accum+3
rts
; add accum to result
;
; result=result+accum+1,2,3
addres clc
ldx #$fd
add100 lda result+3,x
adc accum+4,x
sta result+3,x
inx
bne add100
rts
; .end