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Docs » Autodocs » cd.device » CD_READXL
CD_READXL -- Read from CD-ROM into memory via transfer list.
IO REQUEST
io_Device preset by the call to OpenDevice()
io_Unit preset by the call to OpenDevice()
io_Command CD_READXL
io_Data pointer to transfer list (i.e. struct List *).
io_Length maximum transfer length (WORD multiple) or 0.
io_Offset byte offset from the start of the disk describing
where to read data from, must be a WORD multiple.
io_Error 0 for success, or an error code as described in
<devices/cd.h>
io_Actual if io_Error is 0, number of bytes actually transferred
This command starts reading data off the disk at the specified
location and deposits it into memory according to the nodes in a
transfer list. The pointer to the list of transfer nodes is placed
in io_Data. If you have a non-circular transfer list, simply set
io_Length to 0 (0 is special and means ignore io_Length) -- your
transfer will end when your transfer list has been exhausted. If you
have a circular transfer list, the list will never end. In this case,
the transfer will terminate when io_Length bytes have been
transferred.
The fields in the CDXL node structure are:
struct CDXL {
struct MinNode Node; /* double linkage */
char *Buffer; /* data destination */
LONG Length; /* must be even # bytes */
LONG Actual; /* bytes transferred */
APTR IntData; /* interrupt server data segment */
VOID (*IntCode)(); /* interrupt server code entry */
};
The philosophy here is that you set up the buffers you want filled,
create CDXL nodes describing the locations and sizes of these
buffers, link all the nodes together in the order that you'd like
(even make a circular list for animations), and execute the command.
The data will be streamed into the appropriate buffers until the
list has been exhausted, an entry with a Length of zero is
encountered, io_Length bytes have been transferred (if io_Length is
non-zero), or the command is aborted with AbortIO().
If you fill in the (*IntCode)() field with a pointer to an interrupt
routine, your routine will be called when the transfer for the node
is complete. Your code will be called before the driver proceeds to
the next node. The interrupt should follow the same rules as standard
interrupts (see AddIntServer of Exec autodocs). Register A2 will
contain a pointer to the node just completed. You may manipulate the
list from within the interrupt. Your code must be brief (this is an
interrupt). When returning from this interrupt, D0 should be cleared
and an RTS instruction should be used to return.
Servers are called with the following register conventions:
D0 - scratch
D1 - scratch
A0 - scratch
A1 - server is_Data pointer (scratch)
A2 - pointer to CDXL node just completed
A5 - jump vector register (scratch)
all other registers must be preserved
Try to make sure that small buffers are not overused. Each time
a node is completed, an interrupt is generated. If you find that
your computer is acting sluggish, or the CD_READXL command is
aborting, you are probably generating too many interrupts. It is
not efficient to have more than a few of these interrupts generated
within a vertical blank.
Unlike the READ command, the READXL command will not retry a sector
if there is an error. Since the READXL command's purpose is primarily
for animations, data streaming is considered more important than the
data itself. An error will be returned in io_Error if a data error
did occur. This command will never drop to a lower speed in the event
of an error.
CMD_READ, CD_SEEK(), Autodocs - AddIntServer
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