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Section 3 ½ Computers of Historical Significance 97

The Pc. stack is a major alternative to the main-line organization of 1 address per instruction (augmented with index registers or general registers). It tries to capitalize on the hierarchical character of computation to avoid having to give memory shuffling instructions explicitly. In Chap. 6, we gave a comparison of a trivial computation using a stack and a general-register organization, in order to make clear the case for stacks. However, we did not there attempt any definitive analysis. It has been asserted [Amdahl et al., 1964a] that the Pc. stack derives its power only from its having some fast-working memory in the Pc, thus that it is dominated by the general-register organization. Our own feeling is that the compile and compiled program execution times for the Pc. stack are indeed impressive. However, no definitive analysis has been published, as far as we know. Pc. stack is certainly an organization that rates serious study by any computer designer.

B 5000, B 6500/B 7500, D825, and KDF 9

The PMS structure diagram of the B 5000 and B 6500/B 7500 (Figs 2 to 6) should be compared with Burroughs' own structure representation. The D825 structure is similar. All the Burroughs computers in Table 1 have the multiprocessor structure.

Burroughs was probably the first computer company to take matters of the structure and organization seriously. The D825 hardware and software were designed for military command and control applications, which demand very high uptime and availability. As various computer components in the structures fail, continuous operation is possible at a reduced level through the fail-soft design. However, to our knowledge, no published account exists on how well this design works in practice from a performance and reliability viewpoint.

The structures in the B 6500, especially, allow Kio's to be freely assigned to any T or Ms, thereby achieving better equipment utilization. The S(16 Mp; 16P) is probably overdesigned in the Burroughs B 6500 computers. These structures generally have a maximum 4(P + Kio), although the design is based on 16(P + Kio). The Kio's (Chap. 9) may be overdesigned, as well,, since a K capable of controlling a simple T.card.reader can also control a complex Ms.disk or Ms.magnetic.tape. The comparison of Pc.stack, Pc. 1.address, and Pc.general.register makes the assumption that an unlimited hardware stack resides in Pc. The B 5500 has a local M. stack in Pc of 4 words. The size and number of stacks, and their use by software, are most important, The KDF9 has two independent stacks: one for arithmetic expression evaluation and one for holding subroutine return addresses.

Multiprocessing in the B 6500 is facilitated by the hardware stack features of the architecture. The stack mechanism allows efficient implementation of temporary storage and reentrant software, such as block-structured languages similar to ALGOL. The Segment Dictionary, or Program Reference Table, which was the basis for the Burroughs descriptor method of segmenting in the B 5000, has been moved to the bottom of the tree-structured hardware stack. This allows processes and instantiations of the same program (using different branches of the stack tree) to share

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