Save and load: rentry

       Name: rentry                                                  [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Search the disk catalog for an existing file or an empty file
             entry
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * findf calls rentry


 For a detailed look at how DOS works, see the book "Beneath Apple DOS" by Don
 Worth and Pieter Lechner. In particular, see chapter 4 for the layout of the
 VTOC, catalog sector, file entry and file/track list.


 Arguments:

   C flag               The type of search:

                          * Clear = search the catalog for an existing file

                          * Set = search the catalog for an empty file entry


 Returns:

   C flag               The result of the search:

                          * Clear = file/entry found

                          * Set = file/entry not found

   Y                    The offset to the file entry in the catalog sector


.rentry

 ROR atemp0             ; Store the C flag in bit 7 of atemp0, so we can check
                        ; it later

 JSR rvtoc              ; Read the VTOC sector into the buffer

                        ; We now work through the catalog sectors to look for
                        ; the existing file entry (if bit 7 of atemp0 is clear)
                        ; or an empty file empty (if bit 7 of atemp0 is set)

.rentr2

 LDA buffer+1           ; Set track to the track number of the next catalog
 STA track              ; sector from byte #1 of the VTOC

 LDA buffer+2           ; Set sector to the sector number of the next catalog
 STA sector             ; sector from byte #2 of the VTOC

 JSR rsect              ; Read the catalog sector into the buffer

 LDY #$B                ; Set Y to use as an index to the first file entry in
                        ; the catalog sector (as the file entries start at
                        ; offset $B in the catalog, with each entry taking up
                        ; 35 bytes)

.rentr3

 LDA buffer,Y           ; Set A to the first byte from the file entry, which
                        ; will either be the track number of the file, or 0 to
                        ; indicate an empty file entry, or $FF to indicate a
                        ; deleted file

 BIT atemp0             ; If bit 7 of atemp0 is clear then we are searching the
 BPL rentr4             ; catalog for an existing file entry, so jump to rentr4
                        ; to do this

                        ; If we get here then we are searching for an empty file
                        ; entry

 TAX                    ; If A = 0 then we have just found an empty file entry,
 BEQ rentr6             ; so jump to rentr6 to return from the subroutine with a
                        ; successful result

 CMP #$FF               ; If A = $FF then we have just found a deleted file
 BEQ rentr6             ; entry, so jump to rentr6 to return from the subroutine
                        ; with a successful result

 BNE rentr8             ; This file entry doesn't match our requirements, so
                        ; jump to rentr8 to try the next file entry in this
                        ; catalog sector

.rentr4

                        ; If we get here then we are searching for an existing
                        ; file entry

 TAX                    ; If A = 0 then we have just found an empty file entry,
 BEQ rentr9             ; which means we have not found a match for our file, so
                        ; jump to rentr9 to return from the subroutine with the
                        ; C flag set to indicate that we can't find the file

 CMP #$FF               ; If A = $FF then we have just found a deleted file
 BEQ rentr8             ; entry, which is not a match for our file, so jump to
                        ; rentr8 to try the next file entry in this catalog
                        ; sector

 TYA                    ; Store the file entry index in Y on the stack, so we
 PHA                    ; can retrieve it after the following loop

                        ; We now check the file entry to see if it matches the
                        ; filename in comnam

 LDX #0                 ; Set X = 0 to use as a character index for the filename
                        ; in the file entry

.rentr5

 LDA buffer+3,Y         ; Set A to the Y-th character from the filename in the
 AND #%01111111         ; file entry we are checking (the filename in a file
                        ; entry starts at byte #3)

 CMP comnam,X           ; If the character does not match the X-th character of
 BNE rentr7             ; comnam then the names don't match, to jump to rentr7
                        ; to try the next file entry in this catalog sector

 INY                    ; Increment the character index for the file entry

 INX                    ; Increment the character index for the filename we are
                        ; searching for

 CPX #30                ; Loop back until we have checked all 30 characters
 BNE rentr5

                        ; If we get here then all 30 characters of the filename
                        ; in the file entry match the filename in comnam, so we
                        ; have found the file entry we are looking for

 PLA                    ; Set Y to the file entry index that we stored on the
 TAY                    ; stack above, so it once again points to the entry we
                        ; are checking

.rentr6

 CLC                    ; Clear the C flag to indicate that we have found the
                        ; file entry we are looking for

 RTS                    ; Return from the subroutine

.rentr7

 PLA                    ; Set Y to the file entry index that we stored on the
 TAY                    ; stack above, so it once again points to the entry we
                        ; are checking

.rentr8

 TYA                    ; Set Y = Y + 35
 CLC                    ;
 ADC #35                ; Each file entry in the catalog consists of 35 bytes,
 TAY                    ; so this increments Y to point to the next entry

 BNE rentr3             ; Loop back until we have reached the last file entry

 LDA buffer+1           ; Set track to the track number of the next catalog
                        ; sector from byte #1 of the VTOC

 BNE rentr2             ; If the next catalog sector is non-zero then loop back
                        ; to load and search this sector

                        ; Otherwise we have searched every catalog sector and we
                        ; haven't found what we're looking for, so fall through
                        ; into rentr9 to return from the subroutine with the C
                        ; flag set to indicate that the catalog is full

.rentr9

 SEC                    ; Clear the C flag to indicate that we have not found
                        ; the file entry we are looking for

 RTS                    ; Return from the subroutine