Motorola CPU32 Bedienungsanleitung PDF herunterladen (Seite 100)

100

101

Instruction Set Summary

MOTOROLA M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL 3-29

The inclusion of the NAN data type in the IEEE ﬂoating-point number system requires each

conditional test to include the NAN condition code bit in its Boolean equation. Because a

comparison of a NAN with any other data type is unordered (i.e., it is impossible to determine

if a NAN is bigger or smaller than an in-range number), the compare instruction sets the

NAN condition code bit when an unordered compare is attempted. All arithmetic instructions

also set the NAN bit if the result of an operation is a NAN. The conditional instructions

interpret the NAN condition code bit equal to one as the unordered condition.

The IEEE 754 standard deﬁnes four conditions: equal to (EQ), greater than (GT), less than

(LT), and unordered (UN). In addition, the standard only requires the generation of the

condition codes as a result of a ﬂoating-point compare operation. The FPU can test these

conditions at the end of any operation affecting the condition codes. For purposes of the

ﬂoating-point conditional branch, set byte on condition, decrement and branch on condition,

and trap on condition instructions, the processor logically combines the four FPCC condition

codes to form 32 conditional tests. There are three main categories of conditional tests:

IEEE nonaware tests, IEEE aware tests, and miscellaneous. The set of IEEE nonaware

tests is best used:

• when porting a program from a system that does not support the IEEE standard to a

conforming system, or

• when generating high-level language code that does not support IEEE floating-point

concepts (i.e., the unordered condition).

The 32 conditional tests are separated into two groups; 16 that cause an exception if an

unordered condition is present when the conditional test is attempted and 16 that do not

cause an exception. An unordered condition occurs when one or both of the operands in a

ﬂoating-point compare operation The inclusion of the unordered condition in ﬂoating-point

branches destroys the familiar trichotomy relationship (greater than, equal, less than) that

exists for integers. For example, the opposite of ﬂoating-point branch greater than (FBGT)

is not ﬂoating-point branch less than or equal (FBLE). Rather, the opposite condition is

ﬂoating-point branch not greater than (FBNGT). If the result of the previous instruction was

unordered, FBNGT is true; whereas, both FBGT and FBLE would be false since unordered

fails both of these tests (and sets BSUN). Compiler programmers should be particularly

careful of the lack of trichotomy in the ﬂoating-point branches since it is common for

compilers to invert the sense of conditions.

Table 3-22. FPCC Encodings

Data Type N Z I NAN

+ Normalized or Denormalized 0000

– Normalized or Denormalized 1000

+ 0 0100

– 0 1100

+ Infinity 0010

– Infinity 1010

+ NAN 0001

– NAN 1001

1 2 ... 95 96 97 98 99 100 101 102 103 104 105 ... 645 646

Kommentare zu diesen Handbüchern

Keine Kommentare

Motorola CPU32 Bedienungsanleitung Seite 100

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Prozessoren Motorola CPU32