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Chapter 39 Parallel operation in the Control Data 6600 493

absence of the two restraints. The instruction executions, in comparison, range from three minor cycles for fixed add, 10 minor cycles for floating multiply, to 29 mm r cycles for floating divide.

To provide a relatively continuous source of instructions, one buffer register of 60 bits is located at the bottom of an instruction stack capable of holding 32 instructions (Fig. 5). Instruction words from memory enter the bottom register of the stack pushing up the old instruction words. In straight line programs, only the bottom two registers are in use, the bottom being refilled as quickly as memory conflicts allow. In programs which branch back to an instruction in the upper stack registers, no refills are allowed after the branch, thereby holding the program loop completely in the stack. As a result, memory access or memory conflicts are no longer involved, and a considerable speed increase can be had.

Five memory trunks are provided from memory into the central processor to five of the floating point registers (Fig. 6). One address register is assigned to each trunk (and therefore to the floating point register). Any instruction calling for address register result implicitly initiates a memory reference on that trunk. These instructions are handled through the scoreboard and therefore tend to overlap memory access with arithmetic. For example, a new memory word to be loaded in a floating point register can be brought in from memory but may not enter the register until all previous uses of that register are completed. The central registers, therefore, provide all of the data to the ten functional units, and receive all of the unit results. No storage is maintained in any unit.

Central memory is organized in 32 banks of 4096 words. Consecutive addresses call for a different bank; therefore, adjacent addresses in one bank are in reality separated by 32. Addresses may be issued every 100 nanoseconds. A typical central memory information transfer rate is about 250 million bits per second.

As mentioned before, the functional units are hidden behind the registers. Although the units might appear to increase hard ware duplication, a pleasant fact emerges from this design. Each unit may be trimmed to perform its function without regard to others. Speed increases are had from this simplified design.

As an example of special functional unit design, the floating multiply accomplishes the coefficient multiplication in nine minor cycles plus one minor cycle to put away the result for a total of 10 minor cycles, or 1000 nanoseconds. The multiply uses layers of carry save adders grouped in two halves. Each half concurrently forms a partial product, and the two partial products finally merge while the long carries propagate. Although this is a fairly large complex of circuits, the resulting device was sufficiently smaller than originally planned to allow two multiply units to be included in the final design.

Fig. 5. 6600 instruction stack operation.

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