The Burroughs B6x00-7x00 instruction set includes the set of valid operations for the Burroughs B6500, B7500 and later Burroughs large systems, including the current (as of 2006) Unisys Clearpath/MCP systems; it does not include the instruction for other Burroughs large systems including the B5000, B5500, B5700 and the B8500. These unique machines have a distinctive design and instruction set. Each word of data is associated with a type, and the effect of an operation on that word can depend on the type. Further, the machines are stack based to the point that they had no user-addressable registers. Overview As you would expect from the unique architecture used in these systems, they also have an interesting instruction set. 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. 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. Since there are no programmer-addressable registers, most of the register manipulating operations required in other architectures are not needed, nor are variants for performing operations between pairs of registers, since all operations are applied to the top of the stack. This also makes code files very compact, since operators are zero-address and do not need to include the address of registers or memory locations in the code stream. Some of the code density was due to moving vital operand information elsewhere, to 'tags' on every data word or into tables of pointers. Many of the operators are generic or polymorphic depending on the kind of data being acted on as given by the tag. The generic opcodes required fewer opcode bits but made the hardware more like an interpreter, with less opportunity to pipeline the common cases. For example, the instruction set has only one ADD operator. It had to fetch the operand to discover whether this was an integer add or floating point add. Typical architectures require