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DOS ZERO PAGE USAGE
.SP1
.un
BYTE      USE
24        Cursor horizontal (DOS)
26,27     Sector read buffer address (ROM)
          Scratch space (RWTS)
28,29     BASL/BASH (DOS)
2A        Segment merge counter (ROM,BOOT)
          Scratch space (RWTS)
2B        BOOT slot*16 (ROM)
          Scratch space (RWTS)
2C        Checksum from sector header (RWTS)
2D        Sector number from sector header (RWTS)
2E        Track number from sector header (RWTS)
2F        Volume number from sector header (RWTS)
33        Prompt character (DOS)
35        Drive number in high bit (RWTS)
36,37     CSWL,CSWH (DOS)
38,39     KSWL,KSWH (DOS)
3C        Workbyte (ROM)
          Merge workbyte (BOOT)
          Device characteristics table address (RWTS)
3D        Sector number (ROM)
          Device characteristics table address (RWTS)
3E,3F     Address of ROM sector-read subroutine (BOOT)
          Buffer address (RWTS)
40,41     DOS image address (BOOT)
          File buffer address (DOS)
41        Format track counter (RWTS)
42,43     Buffer address (DOS)
44,45     Numeric operand (DOS)
46,47     Scratch space (RWTS)
48,49     IOB address (RWTS)
4A,4B     INTEGER BASIC LOMEM address (DOS)
          Format diskette workspace (RWTS)
4C,4D     INTEGER BASIC HIMEM address (DOS)
67,68     APPLESOFT BASIC PROGRAM START (DOS)
69,6A     APPLESOFT BASIC VARIABLES START (DOS)
6F,70     APPLESOFT BASIC STRING START (DOS)
73,74     APPLESOFT BASIC HIMEM address (DOS)
76        APPLESOFT BASIC line number high (DOS)
AF,B0     APPLESOFT BASIC PROGRAM END (DOS)
CA,CB     INTEGER BASIC PROGRAM START (DOS)
CC,CD     INTEGER BASIC VARIABLES END (DOS)
D6        APPLESOFT BASIC PROGRAM protection flag (DOS)
D8,D9     INTEGER BASIC line number (DOS)
          APPLESOFT BASIC ONERR (DOS)
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