Save and load: wbyte

       Name: wbyte                                                   [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Write one byte to disk
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * write calls wbyte
             * write calls via wbyte2
             * write calls via wbyte3


 This routine is identical to the WNIBL9 routine in Apple DOS 3.3.

 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.

 Elite uses different label names to the original DOS 3.3 source, but the code
 is the same.

 This code forms part of the RWTS ("read/write track sector") layer from Apple
 DOS, which was written by Randy Wigginton and Steve Wozniak. It implements the
 low-level functions to read and write Apple disks, and is included in Elite so
 the game can use the memory that's normally allocated to DOS for its own use.


 Other entry points:

   wbyte2               Wait for seven cycles before writing instead of nine
                        (this is WNIBL7 in the original source)

   wbyte3               Write straight away without waiting (this is WNIBL in
                        the original source)


.wbyte

 CLC                    ; WNIBL9   CLC (2)        ; 9 CYCLES, THEN WRITE.

.wbyte2

 PHA                    ; WNIBL7   PHA (3)        ; 7 CYCLES, THEN WRITE.
 PLA                    ;          PLA (4)

.wbyte3

 STA Q6H,X              ; WNIBL    STA Q6H,X      ; (5)  NIBL WRITE SUB.
 ORA Q6L,X              ;          ORA Q6L,X      ; (4)  CLOBBERS ACC, NOT
                        ;                                CARRY.
 RTS                    ;          RTS