Intel 80287, 80286 manual Managing Concurrency, Instruction Synchronization

PROGRAMMING NUMERIC APPLICATIONS

No special programming techniques are required to gain the advantages of concurrent execution; numeric instructions for the NPX are simply placed in line with the instructions for the CPU. CPU and numeric instructions are initiated in the same order as they are encountered by the CPU in its instruction stream. However, because numeric operations performed by the NPX generally require more time than operations performed by the CPU, the CPU can often execute several of its instructions before the NPX completes a numeric instruction previously initiated.

This concurrency offers obvious advantages in terms of execution performance, but concurrency also imposes several rules that must be observed in order to assure proper synchronization of the 80286 CPU and 80287 NPX.

All Intel high-level languages automatically provide for and manage concurrency in the NPX. Assembly-language programmers, however, must understand and manage some areas of concurrency in exchange for the flexibility and performance of programming in assembly language. This section is for the assembly-language programmer or well-informed high-level-language programmer.

Managing Concurrency

Concurrent execution of the host and 80287 is easy to establish and maintain. The activities of numeric programs can be split into two major areas: program control and arithmetic. The program control part performs activities such as deciding what functions to perform, calculating addresses of numeric operands, and loop control. The arithmetic part simply adds, subtracts, multiplies, and performs other operations on the numeric operands. The NPX and host are designed to handle these two parts separately and efficiently.

Managing concurrency is necessary because both the arithmetic and control areas must converge to a well-defined state before starting another numeric operation. A well-defined state means all previous arithmetic and control operations are complete and valid.

Normally, the host waits for the 80287 to finish the current numeric operation before starting another. This waiting is called synchronization.

Managing concurrent execution of the 80287 involves _~ree types of synchronization:

1.Instruction synchronization

2.Data synchronization

3.Error synchronization

For programmers in higher-level languages, all three types of synchronization are automatically provided by the appropriate compiler. For assembly-language programmers, instruction synchronization is guaranteed by the NPX interface, but data and error synchronization are the responsibility of the

_____ 1-1 •• 1.... _ ..... ~	"" .,..... ..nt'l'1'""l't'Y1n-,pr
a.",,""JUUJ.y-LUJ. 16U.U.O	.... 1"'" '-'0&	__ .

Instruction Synchronization

Instruction synchronization is required because the 80287 can perform only one numeric operation at a time. Before any numeric operation is started, the 80287 must have completed all activity from its previous instruction.

2-48