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MAS281

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extended 8-bit displacement field within the instruction. This

sum then points to the memory address to which control will be

transferred if the branch is taken.

4.2.7 BASE RELATIVE (B)

There are two formats which implement Base Relative

Addressing; one allows indexing and one does not. For the

non-indexable form, the contents of the instruction specified

base register (BR = BR’ + 12) is added to the 8-bit

displacement field (DU) of the 16-bit instruction. For the

indexable form, the sum of the contents of a specified index

required operand. Registers R1 through R15 may be specified

for indexing. Registers R12 through R15 may be specified as

the base register.

4.2.8 SPECIAL

This addressing mode is applicable to instructions that do

not follow the above formats. The instructions that use this

special mode are indicated in Table 7a.

4.3 BENCHMARKING

Table 7a defines the number and type of machine cycles

associated with each MIL-STD-1750A instruction. This

information may be used when benchmarking MAS281

performance. The Digital Avionics Instruction Set (DAIS) mix,

which defines a typical frequency of occurrence for MIL-STD-

1750A instructions, is used here for this purpose.

One problem with the DAIS mix, however, is that it does not

reflect the impact of data dependencies on system

performance. For example, a multiplication in which an

operand is zero may be performed much faster than one with

two non-zero operands. Also, the DAIS mix does not specify

such time consuming operations as normalization and

alignment.

Realistic benchmarks must therefore take both the

instruction mix and data dependencies into account. To this

end, machine cycle counts in Table 7a which have data

dependencies, are annotated with either an "a" or "wa"suffix.

An "a" suffix reflects an average number of machine

cycles (where each of several possibilities is equally likely) and

“"wa" suffix reflects a weighted average number of machine

cycles (where some data possibilities are more likely than

others).

Weighted averages are only applicable to floating-point

operations. Weighted averages provided in Table 7a are

based on the Sweeney (IBM) guidelines. These guidelines

take a wide range of data dependencies into consideration.

Normalization and alignment operations are also represented.

Table 6 defines MAS281 throughput, at various frequencies

and wait states, for the DAIS mix using Sweeney data

dependencies.

It should be noted that using the Sweeney guidelines is a

conservative approach to benchmarking. If best case

assumptions are made and such operations as normalization

and alignment are not considered, MAS281 performance

figures are approximately 50% higher than those indicated in

Table 6.

743.4

698.3

658.4

622.8

594.7

558.7

526.7

498.2

446.0

419.0

395.0

373.7

297.4

279.3

263.4

249.1

0123

Number of Wait States in Memory Access Cycle

fOSC

MHz

Table 6: Throughput (KIPS)

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