AM2: Protected-Mode Programming

ECP4166: Advanced Microprocessors
AM2
Figure 1.1: Evaluating the Postfix Expression 5 6 * 4 – .
Table 1.1 contains some examples of translating infix to equivalent postfix expressions.
Table 1.1 Infix to Postfix Examples
Infix
Postfix
A + B A B +
(A - B) / D A B - D /
(A + B) * (C + D) A B + C D + *
((A + B) / C) * (E - F) AB + C / EF - *
1.2 FPU Data Registers
The FPU has eight individually addressable 80-bit data registers named R0 through R7 (see
Figure 2). Together, they are called a register stack. A three-bit field named TOP in the FPU
status word identifies the register number that is currently the top of the stack. In Figure 1.2,
for example, TOP equals binary 011, identifying R3 as the top of the stack. This stack
location is also known as ST(0) (or simply ST) when writing floating-point instructions. The
last register is ST(7).
Figure 1.2: Floating-Point Data Register Stack.
As we might expect, a push operation (also called load) decrements TOP by 1 and copies an
operand into the register identified as ST(0). If TOP equals 0 before a push, TOP wraps
around to register R7. A pop operation (also called store) copies the data at ST(0) into an
operand, then adds 1 to TOP. If TOP equals 7 before the pop, it wraps around to register R0.
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