RK06/RK07 Disk Drive User's Manual - Trailing-Edge

2. Error correction is accomplished in six steps:
a. Calculate the byte offset to the start of the block transferred.
b. Calculate offset to the first bad byte in the block.
c. Convert correction pattern into two pattern words.
d. Apply ECC correction using first pattern word.
e. Apply ECC correction using second pattern word.
f. Finish or continue previous function.
DMECC:MOV RKWC,RO ;Get negative words remaining
ASL RO ;Convert to bytes
ADD COUNT,RO ;Calculate bytes actually transfered
MOV RO,-(SP) ;And save it
BEQ DMOFST ;Branch to do offset if no bytes transferred
BIT #HVRC,RKER ;Header VRC error?
BNE 40$ ;Yes
DEC RO ;Calculate offset to start of block
40$: BIC #777,RO ;Clear residue .
BIT #DTE!ECH !HVR C,RKER ;ECC Hard Error?
BNE DMOFST ; Y es do offset
MOV RKECPS,Rl ;Get starting bit number
DEC Rl ;Convert to relative bit number
MOV Rl,-(SP) ;Save starting relative bit number
BIC #tC,Rl ;Isolate shift count
BIC Rl,(SP) ;Clear shift count in relative bit number
CLC ;Calculate offset to first bad byte in ....
ROR (SP) ; ... the block
ASR (SP)
ASR (SP)
ADD (SP),RO ;Set offset to first byte in the transfer
MOV RKECPT,R3 ;Get correction pattern word
CLR (SP) ;Clear second pattern word
41$: DEC Rl ;Any shifts to perform?
BLT 42$ ;IfLTno
ASL R3 ;This is in case the error burst. ..
ROL (SP) ; ... crossed a 16-Bit word boundary
BR 41$ ;Continue shifting
42$: CALL DMECOR ;Apply ECC correction
MOV (SP)+,R3 ;Retrieve second pattern word
ADD #2,RO ;Update offset to next bad(?) Word
CALL DMECOR ;Apply ECC correction
;Correction completed
;Finish or continue function
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