AM2: Protected-Mode Programming

ECP4166: Advanced Microprocessors
AM2
1.3.1 Operands
A floating-point instruction can have zero operands, one operand, or two operands. If there
are two operands, one must be a floating-point register. There are no immediate operands, but
certain predefined constants (such as 0.0, _and log2 10) can be loaded into the stack.
General-purpose registers such as EAX, EBX, ECX, and EDX cannot be operands. (The only
exception is FSTSW, which stores the FPU status word in AX.) Memory-to-memory
operations are not permitted. Integer operands must be loaded into the FPU from memory
(never from CPU registers); they are automatically converted to floating-point format.
Similarly, when storing floating-point values into integer memory operands, the values are
automatically truncated or rounded into integers.
1.3.2 Initialization (FINIT)
The FINIT instruction initializes the FPU. It sets the FPU control word to 037Fh, which
masks (hides) all floating-point exceptions, sets rounding to nearest even, and sets the
calculation precision to 64 bits. We recommend calling FINIT at the beginning of your
programs, so you know the starting state of the processor.
1.3.3 Floating-Point Data Types
Let’s quickly review the floating-point data types supported by MASM (QWORD, TBYTE,
REAL4, REAL8, and REAL10), listed in Table 1.2. You will need to use these types when
defining memory operands for FPU instructions. For example, when loading a floating-point
variable into the FPU stack, the variable is defined as REAL4, REAL8, or REAL10:
.data
bigVal REAL10 1.212342342234234243E+864
.code
fld bigVal ; load variable into stack
Table 1.2: Intrinsic Data Types.
Type Usage
QWORD 64-bit integer
TBYTE 80-bit (10-byte) integer
REAL4 32-bit (4-byte) IEEE short real
REAL8 64-bit (8-byte) IEEE long real
REAL10 80-bit (10-byte) IEEE extended real
1.3.4 Reading and Writing Floating-Point Values
In this experiment, the following two procedures for floating-point input-output (created by
William Barrett of San Jose State University) are used:
ReadFloat: Reads a floating-point value from the keyboard and pushes it on the floatingpoint
stack. It accepts a wide variety of floating-point formats. Some examples
are shown below:
35 3.5E005
+35. -3.5E+5
-3.5 3.5E-4
.35 +3.5E-4
3.5E5
WriteFloat: Writes the floating-point value at ST(0) to the console window in exponential
format.
ShowFPUStack: Another useful procedure, written by James Brink of Pacific Lutheran
University, displays the FPU stack. It is called with no parameters:
call ShowFPUStack
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