references are through the 1024 word Program Reference Table, current code segment, marked locations within the stack or to the A and B registers holding the top two locations on the stack. Burroughs numbers bits in a syllable from 0 (high bit) to 11 (low bit) B6500, B7500 and successors Programs are made up of 8-bit syllables, which may be Name Call, be Value Call or form an operator, which may be from one to twelve syllables in length. There are less than 200 operators, all of which fit into 8-bit syllables. Many of these operators are polymorphic depending on the kind of data being acted on as given by the tag. If we ignore the powerful string scanning, transfer, and edit operators, the basic set is only about 120 operators. If we remove the operators reserved for the operating system such as MVST and HALT, the set of operators commonly used by user-level programs is less than 100. The Name Call and Value Call syllables contain address couples; the Operator syllables either use no addresses or use control words and descriptors on the stack. Multiple processors The B5000 line also were pioneers in having multiple processors connected together on a high-speed bus. The B7000 line could have up to eight processors, as long as at least one was an I/O module. RDLK is a very low-level way of synchronizing between processors. The high level used 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. Notable operators are: HEYU — send an interrupt to another processor 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. x:=RDLK(x,y); WHOI — Processor identification 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 deadly embrace'. Influence of the B5000 The direct influence of the B5000 can be seen in the current Unisys ClearPath range of