Specifications

Operating System Routines

IOC$ALLOCATE_CRAM

Allocates a control register access mailbox (CRAM).

Module

[SYSLOA]CRAM_ROUTINES_LSB

Input

Location Contents

04(SP) Reserved for returned address of CRAM

Output

Location Contents

04(SP) Address of CRAM

R0 Status of operation

Synchronization

During normal processing, IOC$ALLOCATE_CRAM uses no spinlocks. However,

if there are no free CRAMs available, the routine acquires and releases the

MMG spinlock while allocating additional CRAMs from nonpaged memory. Thus,

the caller should be running at or below IPL$_MMG. If the caller is running

above IPL$_MMG, no attempt is made to allocate additional CRAMs, and

IOC$ALLOCATE_CRAM returns an error status.

Description

IOC$ALLOCATE_CRAM ﬁrst checks to see if memory has already been allocated

for use as mailboxes. If memory has been allocated, the routine checks to see if a

CRAM is available. If no CRAMs have been conﬁgured or none are available, the

routine allocates four pages of system nonpaged memory and divides this memory

into one CRAM header (CRAMH) and 15 CRAMs.

When an available CRAM is found, the routine marks it as unavailable and

returns its address to the caller.

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