Instruction manual

READ
READ
Read from an I/O device.
Format
status = DISPATCH(READ,
channel_nbr,
byte_count,
buffer_ptr,
block);
Arguments
READ
READ function code (13
16
).
channel_nbr
The channel number of the device to access.
byte_count
The number of bytes to read from the device.
buffer_ptr
The virtual address of the buffer into which the data is read.
block
The location from which to read the data on the device. For a disk device this
argument is a logical block number; for FLASH ROM this argument is a logical
section of FLASH ROM.
Description
This routine reads multiple bytes from an I/O device associated with channel_nbr
and places the read data in the buffer pointed to by buffer_ptr. The number of
bytes read must be a multiple of the device’s record length. If this number is not
a multiple of the record length, the routine rounds the given number down to the
nearest multiple.
For disk devices, READ does not understand the file structure of the device and
reads contiguous physical blocks. For network devices, READ determines if a
network packet addressed specifically to this DEC 3000 AXP has been received.
If so, the routine copies the packet to the user’s buffer. The size of the packet is
device- and protocol-dependent. The entire packet, including header information,
is returned to the caller. The Ethernet source address is inserted into the source
bytes in the packet.
16–66 Console