745



     have created a MEM.SAV file, the data will be wiped out when
     you call DOS. To alter MEMLO, you start by POKEing WARMST
     (location 8) with zero, then doing a JMP to the BASIC cartridge
     entry point at 40960($A000) after defining your area to protect.
     For example, try this:

     10 DIM MEM$(24):PROTECT=700:REM NUMBE
        R OF BYTES TO CHANGE
     15 HIBYTE=INT(PROTECT/256):LOBYTE=PRO
        TECT-256*HIBYTE
     20 FOR N=1 TO 24:READ PRG:MEM$(N)=CHR
        $(PRG):NEXT N
     30 MEM$(6,6)=CHR$(LOBYTE):MEM$(14,14)
        =CHR$(HIBYTE)
     40 RESERVE=USR(ADR(MEM$))
     50 DATA 24,173,231,2,105,0,141,231,2,
        173,232,2,105
     60 DATA 0,141,232,2,169,0,133,8,76,0,
        160 

     You will find the address of your reserved memory by: PRINT
     PEEK(743) + PEEK(744) * 256 before you run the program. This
     program will wipe itself out when run. Altering MEMLO is the
     method used by both DOS and the RS-232 port driver in the 850
     Interface. See COMPUTE!, July 1981.

745           2E9             ....
     Spare byte.

746-749        2EA-2ED        DVSTAT
     Four device status registers used by the I/O status operation as
     follows:

     746 ($2EA) is the device error status and the command status
     byte. If the operation is a disk I/O, then the status returned is that
     of the 1771 controller chip in your Atari disk drive. Bits set to one
     return the following error codes:

     Bit  Decimal  Error
     0       1     An invalid command frame was received (error).
     1       2     An invalid data frame was received.
     2       4     An output operation was unsuccessful.
     3       8     The disk is write-protected.
     4      16     The system is inactive (on standby).
     7      32     The peripheral controller is "intelligent" (has its
                   own microprocessor: the disk drive). All Atari
                   devices are intelligent except the cassette
                   recorder, so BIT 7 will normally be one when a
                   device is attached.

     747 ($2EB) is the device status byte. For the disk, it holds the