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QBasic FAQ

The Ultimate Method for Writing Memory to a File
Posted: 15 Jul 00

    With all of the questions about accessing memory with QB I decided to show everyone the ultimate method for saving large blocks of RAM to a file.

    A conventional loop setting memory segments and then PEEKing at each byte in each 16 byte segment could require many many minutes to read and write the RAM that this program accesses in a fraction of a second. It appears to be several times faster (and more flexible) than the BSAVE command.

How fast? When this program is timed, using Interrupt 21 function 2c, running on a K62/350, it comes in at .05 seconds. It's hard to get a more accurate time with DOS since the system timer resolution is .05 seconds. In any case, this little routine gets and saves one megabyte of memory in very short order.

    You will need QB45 or better and must load the QB library (or equivalent) with the command line "QB /L QB.QLB".

' $DYNAMIC
DEFINT A-Z

'Declare the registers. Don't use ' $INCLUDE: QB.BI to load
'them. This one requires that DS is a long integer.
'Don't try to modify the other registers or use this
'declaration in a different program.

TYPE RegTypeX
            AX      AS INTEGER
            BX      AS INTEGER
            CX      AS INTEGER
            DX      AS INTEGER
            BP      AS INTEGER
            SI      AS INTEGER
            DI      AS INTEGER
            FLAGS   AS INTEGER
            DS      AS LONG
            ES      AS INTEGER
END TYPE
DIM InRegs AS RegTypeX, OutRegs AS RegTypeX

'MemFile$ will contain the name of the file we want to write.
MemFile$ = "1STMEG" + CHR$(0)
InRegs.AX = &H3C00
InRegs.CX = &H20
InRegs.DS = VARSEG(MemFile$)
InRegs.DX = SADD(MemFile$)
CALL InterruptX(&H21, InRegs, OutRegs)
'Create an empty file
InRegs.AX = &H3D01
'01 = open the file for write access
InRegs.DS = VARSEG(MemFile$)
InRegs.DX = SADD(MemFile$)
CALL InterruptX(&H21, InRegs, OutRegs)
'Open the file
hFile = OutRegs.AX 'hFile is the handle for the file

FOR Segment& = 0 TO 65535 STEP 2048
'Skip segs already read
    InRegs.AX = &H4000
    InRegs.BX = hFile

    InRegs.CX = 32767 'Read this many bytes
    InRegs.DS = Segment& 'Begin with this segment
    InRegs.DX = 0 'Start at offset 0
    'Write the bytes to the file
    CALL InterruptX(&H21, InRegs, OutRegs
NEXT
InRegs.AX = &H3E00
InRegs.BX = hFile
CALL InterruptX(&H21, InRegs, OutRegs)
'Close the file

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