Section 5 Processors with stack memories (zero addresses per instruction) 259

Fig. 3. Burroughs B 6500, B 7500 Ms (diSk) PMS diagrams.

for stacks. However, we did not there attempt any 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.

The PMS structure diagram of the B 5000 and B 6500/B 7500 (Figs 1 to 5) should be compared with Burroughs own structure representation (Chap. 22, page 268). The D825 structure is similar; it is given in Chap. 36, page 447. 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 philosophy and details of the D825 software and hardware are discussed in Chap. 36.

The structures in the B 6500, especially, allow Kios to be freely assigned to any T or Ms, thereby achieving better equipment utilization. The S(16 Mp; 16 P) 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 Kios (Chap. 22) may be overdesigned, too, since a K capable of controlling a simple T.card_ reader can also control a complex Ms.disk or Ms.magnetic_ tape.

The PMS structure of the English Electric KDF9 (Fig. 6) is fairly simple. The 16 K's for direct memory access appear