Editing Burroughs Large Systems (section)

==Multiple processors==
The B5000 line also were pioneers in having two processors connected together on a high-speed bus as master and slave. In the B6000, B7000 and B8000 lines the processors were symmetric. The B7000 line could have up to eight processors, as long as at least one was an I/O module. '''RDLK''' (ReaD with LocK) is a very low-level way of synchronizing between processors. '''RDLK''' operates in a single cycle. The higher level mechanism generally employed by user programs is the '''EVENT''' data type. The '''EVENT''' data type did have some system overhead. To avoid this overhead, a special locking technique called Dahm locks (named after a Burroughs software guru, Dave Dahm) can be used.  Dahm locks used the '''READLOCK''' ALGOL language statement which generated a '''RDLK''' operator at the code level.

Notable operators are:

'''HEYU''' — send an interrupt to another processor<br />
'''RDLK''' — Low-level semaphore operator: Load the A register with the memory location given by the A register and place the value in the B register at that memory location in a single uninterruptible cycle. The Algol compiler produced code to invoke this operator via a special function that enabled a "swap" operation on single-word data without an explicit temporary value. <code>x:=RDLK(x,y);</code><br />
'''WHOI''' — Processor identification<br />
'''IDLE''' — Idle until an interrupt is received

Two processors could infrequently simultaneously send each other a 'HEYU' command resulting in a lockup known as 'a [[Deadlock (computer science)|deadly embrace]]'.