Save and load: rttrk

       Name: rttrk                                                   [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Read or write a sector on the current track
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * rdrght calls rttrk


.rttrk

 LDY sector             ; Use the scttab lookup table to set A to the physical
 LDA scttab,Y           ; sector number of logical sector Y

 CMP idfld+1            ; If the physical sector number doesn't match the sector
 BNE trytr4             ; ID, jump to trytr4 to try reading the track again

 PLP                    ; Fetch the read/write status into the C flag from the
                        ; stack

 BCS rttrk2             ; If the C flag is set then we are writing a sector, so
                        ; jump to rttrk2 to write the sector to the disk

 JSR read               ; Otherwise we are reading a sector, so call the read
                        ; routine to read the current sector into the buffer at
                        ; buffr2, which will load the entire commander file as
                        ; it fits into one sector
                        ;
                        ; Note that this loads the file straight from disk, so
                        ; it is in the 6-bit nibble format

 PHP                    ; Store the status flags on the stack, so if we take the
                        ; following branch, the stack will be in the correct
                        ; state, with the read/write status on top

 BCS trytr4             ; If there was an error then the read routine will have
                        ; set the C flag, so if this is the case, jump to trytr4
                        ; to try reading the track again

 PLP                    ; Otherwise there was no error, so pull the status flags
                        ; back off the stack as we don't need them there any
                        ; more

 JSR pstnib             ; Call pstnib to convert the sector data that we just
                        ; read into 8-bit bytes, processing the 6-bit nibbles in
                        ; buffr2 into 8-bit bytes in buffer

 JMP rttrk3             ; Jump to rttrk3 to return from the RWTS code with no
                        ; error reported

.rttrk2

 JSR write              ; This does nothing except clear the C flag, as we do
                        ; not need to write anything to disk in the game loader

 LDA #1                 ; Set A = 1 to return as the error number for the "Disk
                        ; write protected" error

 BCS rttrk4             ; This beanch is never taken as the call to write clears
                        ; the C flag

                        ; Fall through into rttrk3 to successfully return from
                        ; the RWTS code with no error reported