Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 8-3

PROGRAMMING WITH THE X87 FPU

The x87 FPU instructions treat the eight x87 FPU data registers as a register stack (see

Figure 8-2). All addressing of the data registers is relative to the register on the top

of the stack. The register number of the current top-of-stack register is stored in the

TOP (stack TOP) field in the x87 FPU status word. Load operations decrement TOP

by one and load a value into the new top-of-stack register, and store operations

store the value from the current TOP register in memory and then increment TOP by

one. (For the x87 FPU, a load operation is equivalent to a push and a store operation

is equivalent to a pop.) Note that load and store operations are also available that

do not push and pop the stack.

Figure 8-1. x87 FPU Execution Environment

Data Registers

Exponent Significand

64 63

Control

Status

Tag

Last Instruction Pointer

Last Data (Operand) Pointer

Opcode

Sign