Condition
C3
C2
C0
ST(0) > 0.0
0
0
0
ST(0) < 0.0
0
0
1
ST(0) 
0.0
1
0
0
Unordered
1
1
1
Opcode |
Instruction |
Description |
|
D9 E4 |
FTST |
Compare ST(0) with 0.0. |
Compares the value in the ST(0) register with 0.0 and sets the condition code flags C0, C2, and C3 in the FPU status word according to the results (see table below).
Condition |
C3 |
C2 |
C0 |
|
ST(0) > 0.0 |
0 |
0 |
0 |
|
ST(0) < 0.0 |
0 |
0 |
1 |
|
ST(0) |
1 |
0 |
0 |
|
Unordered |
1 |
1 |
1 |
This instruction performs an "unordered comparison." An unordered comparison also checks the class of the numbers being compared (see FXAM--Examine ). If the value in register ST(0) is a NaN or is in an undefined format, the condition flags are set to "unordered" and the invalid operation exception is generated.
The sign of zero is ignored, so that -0.0 +0.0.
CASE (relation of operands) OF
Not comparable: C3, C2, C0 111;
ST(0) > 0.0: C3, C2, C0 000;
ST(0) < 0.0: C3, C2, C0 001;
ST(0) 0.0: C3, C2, C0 100;
ESAC;
C1 - Set to 0 if stack underflow occurred; otherwise, cleared to 0.
C0, C2, C3 - See above table.
#IS - Stack underflow occurred.
#IA - The source operand is a NaN value or is in an unsupported format.
#D - The source operand is a denormal value.
#NM - EM or TS in CR0 is set.
#NM - EM or TS in CR0 is set.
#NM - EM or TS in CR0 is set.