Specifications

Device Driver Entry Points

Start-I/O Routine

Activates a device to process a requested I/O function.

Speciﬁed in

Specify the name of the start-I/O routine in the start argument of the DDTAB

macro. This macro places the address of the routine into DDT$L_START.

Called by

The start-I/O routine is called by IOC$INITIATE and IOC$REQCOM in module

IOSUBNPAG.

Synchronization

A start-I/O routine is placed into execution at fork IPL, holding the associated

fork lock in a multiprocessing environment. It must relinquish control of the

processor in the same context.

For many devices, the start-I/O routine raises IPL to IPL$_POWER to check

that a power failure has not occurred on the device prior to loading the device’s

registers. The start-I/O routine initiates device activity at device IPL, after

acquiring the corresponding device lock in a multiprocessing environment. An

invocation of the WFIKPCH or WFIRLCH macro to wait for a device interrupt

releases this device lock.

Context

Because a start-I/O routine gains control of the processor in the context of a fork

process, it can refer only to those addresses that reside in system (S0) space.

A start-I/O routine must preserve the contents of all registers except R0, R1, R2,

and R4. If the start-I/O routine uses the stack, it must restore the stack before

completing the request, waiting for an interrupt, or requesting system resources.

Input

Location Contents

R3 Address of IRP

R5 Address of UCB

UCB$W_BCNT Number of bytes to be transferred, copied from the

low-order word of IRP$L_BCNT

UCB$W_BOFF Byte offset into ﬁrst page of direct-I/O transfer;

for buffered-I/O transfers, number of bytes to be

charged to the process allocating the buffer

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