APPENDIX ELEVEN



     Byte  Use
     125   Leftmost six bits: file number (0-63, $3F);
           rightmost two bits: next sector number (high two
           bits)
     126   Next sector number (low eight bits of the sector
           number)
     127   Number of bytes used in this sector (0-125, $7D)

     The next sector to read is in a ten-bit number: eight bits from
     byte 126 ($7E) and the two low bits of 125 ($7D). This means the
     six leftmost bits remaining in byte 125 can be used only to count
     up to 63 (which with zero makes for 64 filenames in one direc-
     tory). This is true when reading linked files, such as BASIC pro-
     grams or text files; auto-boot programs are usually sequential
     and are not linked in this manner (nor are the first four boot
     sectors, the VTOC, or directory sectors). When the next sector
     number is zero, there are no more sectors to read.

     A binary file always begins with 255 ($FF) twice, then four
     bytes: the LSB and MSB ot the start and end addresses, respec-
     tively, of the data to follow (that is, if they were 00 A0 00 B0, it
     would start at $A000 and end at $B000). When a number of
     bytes are loaded to fulfill the load vector, DOS assumes the next
     four bytes are more start/end address vectors and will continue
     to input the following data at the new address unless an EOF
     (End Of File) is reached. Control is passed back to DOS at the
     end of a load unless you put a new run address into 736,737
     ($2E0, $2E1). You can append a code like E0 02 E1 02 00 A0 to
     your binary file (tour address bytes, tollowed by the appro-
     priate data--two bytes to fill the two locations specified), which
     in this case makes the new run address $A000. See COMPUTE',
     March 1982.

1801           709            SABYTE
     Can be set greater than 7, but it only wastes memory space.

1923           783            ....
     Stores the drive number for the DUPSYS file. It you POKE here
     with the ASCII equivalent of the drive number (for example,
     POKE 1923,50 for drive 2), when you call DOS from BASIC,
     DUP.SYS will be loaded trom the drive specified rather than the
     default D1:. To make a permanent change to your DOS, POKE
     the appropriate number, go to DOS, and write DOS files to a
     disk.