mb(9r) mb(9r)
NAME
mb - General: Performs a memory barrier
SYNOPSIS
void mb(
void );
ARGUMENTS
None
DESCRIPTION
The Alpha architecture does not guarantee read/write ordering. That is, the memory subsystem is free to complete read and write operations
in any order that is optimal, without regard for the order in which they were issued. Read/write ordering is not the same as cache
coherency, which is handled separately and is not an issue. The Alpha architecture also contains a write buffer (as do many high-perfor-
mance RISC CPUs, including the MIPS R3000). This write buffer can coalesce multiple writes to identical or adjacent addresses into a single
write, effectively losing earlier write requests. Similarly, multiple reads to the same identical or adjacent addresses can be coalesced
into a single read.
This coalescing has implications for multiprocessor systems, as well as systems with off-board I/O or DMA engines that can read or modify
memory asynchronously or that can require multiple writes to actually issue multiple data items. The mb (memory barrier) routine guarantees
ordering of operations. The mb routine is derived from the MB instruction, which is described in the Alpha Architecture Reference Manual.
The mb routine is a superset of the wbflush routine that ULTRIX drivers use. For compatibility, wbflush is aliased to mb on Tru64 UNIX
Alpha systems.
You call mb in a device driver under the following circumstances: To force a barrier between load/store operations After the CPU has pre-
pared a data buffer in memory and before the device driver tries to perform a DMA out of the buffer Before attempting to read any device
CSRs after taking a device interrupt Between writes
Device drivers and the operating system are the primary users of the mb routine. However, some user programs, such as a graphics program
that directly maps the frame buffer and manipulates registers, might need to call mb. The operating system does not provide a C library
routine for mb. User programs that require use of mb should use the following asm construct:
#include <c_asm.h>
asm ("mb");
NOTES
In most situations that would require a cache flush on other CPU architectures, you should call the mb routine on Tru64 UNIX Alpha systems.
The reason is not that mb is equivalent to a cache flush (as it is not). Rather, a common reason for doing a cache flush is to make data
that the host CPU wrote available in main memory for access by the DMA device or to access from the host CPU data that was put in main mem-
ory by a DMA device. In each case, on an Alpha CPU you should use a memory barrier to synchronize with that event.
One example of using mb occurs with an Ethernet network controller. Each Ethernet network controller has a unique Ethernet hardware address
that is typically contained in a ROM on the Ethernet controller board. The Ethernet hardware address is a multibyte sequence typically con-
sisting of at least 10 bytes. This multibyte Ethernet hardware address is frequently read from the controller hardware by the driver's
probe routine by issuing a sequence of reads to the same controller register. Each successive read returns the next byte of the Ethernet
hardware address. In such instances, a call to mb should be inserted between each of these read operations to ensure that successive read
operations are not coalesced into fewer actual reads as seen by the Ethernet controller.
RETURN VALUES
None
mb(9r)