Chapter 16

Burroughs' B6500/B7500 Stack Mechanism¹

E. A. Hauck / B. A. Dent

Introduction

Burroughs' B6500/B7500 system structure and philosophy are an extention of the concepts employed in the development of the B5500 system. The unique features, common to both hardware systems, are that they have been designed to operate under the control of an executive program (MCP) and are to be programmed in only higher level languages (e.g., ALGOL, COBOL, and FORTRAN). Through a close integration of the software and hardware disciplines, a machine organization has been developed which permits the compilation of efficient machine code and which is addressed to the solution of problems associated with multiprogramming, multiprocessing and time sharing.

Some of the important features provided by the B6500/B7500 system are dynamic storage allocation, re-entrant programming, recursive procedure facilities, a tree structured stack organization, memory protection and an efficient interrupt system. A comprehensive stack mechanism is the basic ingredient of the B6500/ B7500 system for providing these features.

B6500/B7500 Processor

The command structure of the B6500/B7500 Processor is Polish string, which allows for the separation of program code and data addresses. The basic machine instruction is called an operator syllable. This operator syllable is variable in length, from a minimum of 8 bits to a maximum of 96 hits. In the interest of code compactness, more frequently used operator syllables are encoded in the 8 bit form.

The Processor is provided with a hardware implemented stack in which to manipulate data and store dynamic program history. Also, data may be located in arrays outside the stack and may be brought to the stack temporarily for processing. Program parameters, local variables, references to program procedures and data arrays are normally stored within the stack.

The data word of the B6500/B7500 Processor is 51 bits long. Data are transferred between memory and within the Processor in 51 bit words. The first 3 bits of the word are used as tag bits, which serve to identify the various word types as illustrated in Fig. 1. The remaining 48 bits are data. Tag bits, in addition to identifying word type, provide the B6500/B7500 Processor with two unique features: (1) data may be referenced as an operand, with the processor worrying about whether the operand consists of one or two words, and (2) system integrity and memory protection are extended to the level of the basic machine data words. If a job attempts to execute data as program code, or to modify program code, the system is interrupted.
 
 

The Stack

The stack consists of an area of memory assigned to a job. This stack area serves to provide storage for basic program and data references associated with the job. In addition, it provides a facility for the temporary storage of data and job history. When the job is activated, four high speed registers (A, X, B and Y) are linked to the job's stack area (Fig. 2). This linkage is established by the stack pointer register (5), which contains the memory address of the last word placed in the stack memory area. The four top-of-stack registers (A, X, B and Y) function to extend the job's stack into a quick access environment for data manipulation.

Data are brought into the stack through the top-of-stack registers. The stack's operating characteristic is such that the last operand placed into the stack is the first to be extracted. The top-of-stack registers become saturated after having been filled with two operands. Loading a third operand into the top-of-stack

¹SJCC, 1968, pp. 245-251.
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