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294 Part 2½ Regions of Computer Space Section 3½ Concurrency: Single-Processor System


context of the Model 91 organization will be reviewed here. The instruction unit, in preparing instructions for the floating-point operation stack (FLOS), maps both storage-to-register and register-to-register instructions into a pseudo-register-to-register format. In this format R1is always one of the four floating-point registers (FLR) defined by the architecture. It is usually the sink of the instruction, i.e., it is the FLR whose contents are set equal to the result of the operation. Store operations are the sole exception¹ wherein R1 specifies the source of the operand to be placed in storage. A word in storage is really the sink of a store. (R1 and R2 refer to fields as defined by System/360 architecture.)

In the pseudo-register-to-register format "seen" by the FLOS the R2 field can have three different meanings. It can be an FLR as in a normal register-to-register instruction. If the program contains a storage-to-register instruction, the R2 field designates the floating-point buffer (FLB) assigned by the instruction unit to receive the storage operand. Finally, R2 can designate a store data buffer (SDB) assigned by the instruction unit to store instructions. In the first two cases R2 is the source of an operand; in the last case it is a sink. Thus, the instruction unit maps all of storage into the 6 floating-point buffers and 3 store data buffers so that the FLO S sees only pseudo-register-to-register operations.

The distinction between source and sink will become quite important during the discussion of precedence and should be fixed firmly in mind. All of the instructions (except store and compare) have the following form:

For example, the instruction AD 0, 2 means "place the double-precision sum of registers 0 and 2 in register 0," i.e., R0 + R2 ® R0. Note that RI is really both a source and a sink.² Nevertheless, it will be called the sink and R2 the source in all subsequent discussion.

This definition of operations and the machine organization taken together imply a set of data registers with transfer paths among them. These are shown in Fig. 1. The major sets of registers (FLR's, FLB's, FLOS and SDB's) have already been discussed, both above and in Anderson, Sparacio, and Tomasulo [1967]. Two additional registers, one sink and one source, are shown feeding each execution circuit. Initially these registers were considered to be the internal working registers required by the execution circuits and put to multiple use in a way to be described below. Later, their function was generalized under the reservation station concept and they were dissociated from their "working" function.

In actually designing a machine the data paths evolve as the design progresses. Here, however, a complete, first-pass data path will be shown to facilitate discussion. To illustrate the operation let us consider, in turn, four kinds of instructions-load of a register from storage, storage-to-register arithmetic, register- to-register arithmetic, and store. Let us first see how each can be accomplished in vacuo; then what difficulties arise when each is embedded in the context of a program. For simplicity double- precision (64-bit operands) will be used throughout.

Figure 2 shows the timing relationship between the instruction unit's handling of an instruction and its processing by the FLOS decode. When the FLOS decodes a load, the buffer which will receive the operand has not yet been loaded from storage.³ Rather than holding the decode until the operand arrives, the FLOS sets control bits associated with the buffer which cause its content to be transmitted to the adder when it "goes full." The adder receives control information which causes it to send data to floating-point register R1, when its source register is set frill by the buffer.

If the instruction is a storage-to-register arithmetic function, the storage operand is handled as in load (control bits cause it to be forwarded to the proper unit) but the floating-point register, along with the operation, is sent by the decoder to the appropriate unit. After receiving the buffer the unit will execute the operation and send the result to register R1.

In register-to-register arithmetic instructions two floating point registers are transmitted on successive cycles to the appropriate execution unit.

Stores are handled like storage-to-register arithmetic functions, except that the content of the floating-point register is sent to a store data buffer rather than to an execution unit.

Thus far, the handling of one instruction at a time has proven rather straightforward. Now consider the following "program":

The load can be handled as before, but what about the multiply? Certainly F0 and FLB2 cannot be sent to the multiplier as in the case of the isolated multiply, since FLB1 has not yet been set into F0.⁴ This sequence illustrates the cardinal precedence principle: