using the bootstrap loader

WRITING A CUSTOM FIRST-STAGE DRIVER
5.3 DEVICE READ PROCEDURE
5-4
The device read procedure must present the following PL/M -86 interface to the Bootstrap
Loader:
device$read: PROCEDURE (unit, blk$num, buf$ptr) PUBLIC;
DECLARE unit WORD,
blk$num DWORD,
buf$ptr POINTER;
END device$read;
where:
(code as described below)
device$read The name of the device read procedure. You can choose any name you
wish for this procedure, as long as it does not conflict with the names of
any other first-stage procedure.
unit The device unit number, as defined during Bootstrap Loader
configuration.
blk$num
buf$ptr
A 32-bit number specifying the block number the Bootstrap Loader
wants the procedure to read. The size of each block equals the device
granularity, with the first block on the device being block number o.
A 32-bit POINTER to the buffer in which the device read procedure
must copy the information it reads from the secondary storage device.
The device read procedure does not return a value to the caller.
To be compatible with the Bootstrap Loader, the device read procedure must do the
following steps:
1. Read the block specified by blk$num from the bootstrap device specified by unit into
the memory location specified by buf$ptr.
2. Check for I/O errors. If none occur, return to the caller. Otherwise, combine the
device code, if any, for the device with 01 (in the form 01), push the
resulting word value onto the stack, and call the BSERROR procedure. For example,
if the device code is OB3H, push B301H onto the stack, and call BSERROR. If no
device code exists, use 00.
Bootstrap Loader
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