Propeller Manual

CMPSX – Assembly Language Reference
2
The C flag result of CMPSX (Compare Signed, Extended) may differ from CMPX (Compare Unsigned, Extended) where the
“interpreted sign” of Source and Destination are opposite. The first example in the second group, above, shows that CMPSX sets C
because signed $8000_0000 (-2,147,483,648) is
less than signed $7FFF_FFFF (2,147,483,647). CMPX, however, would have cleared
C because unsigned $8000_0000 (2,147,483,648) is not
less than unsigned $7FFF_FFFF (2,147,483,647). The second example is the
complementary case where the Source and Destination values are switched. Note that examples with differing Z and C are not
shown but have expected effects similar to the other examples.
3
The examples of the third group, above, demonstrate cases where the comparison is properly reflected in the flags but the
Destination Out has crossed the signed border (signed overflow error) in either the negative or positive direction. This signed
overflow condition can not be reflected in the flags. If this condition is important to an application, it must detect it through other
means.
Explanation
CMPSX (Compare Signed, Extended) compares the signed values of SValue1 and SValue2 plus
C. The Z and C flags, if written, indicate the relative equal, and greater or lesser relationship
between the two. The
CMPSX instruction is used to perform signed multi-long comparison;
64-bit comparisons, for example.
In a signed multi-long operation, the first instruction is unsigned (ex:
CMP), any middle
instructions are unsigned, extended (ex:
CMPX), and the last instruction is signed, extended (ex:
CMPSX). Make sure to use the WC, and optionally WZ, effect on all the instructions in the
comparison operation.
For example, a signed double-long (64-bit) comparison may look like this:
cmp XLow, YLow wc wz 'Compare low longs; save C and Z
cmpsx XHigh, YHigh wc wz 'Compare high longs; save C and Z
After executing the above, the C and Z flags will indicate the relationship between the two
double-long (64-bit) values. If XHigh:XLow started out as $FFFF_FFFF:FFFF_FFFF (-1)
and YHigh:YLow was $0000_0000:0000_0001 (1) the resulting flags would be: Z = 0 and
C = 1; (Value1 < Value2). This is demonstrated below. Note that the comparison is really
just a subtraction with the result not written; the Z and C flag result is important, however.
Hexadecimal Decimal Flags
(high) (low)
(XHigh:XLow) $FFFF_FFFF:FFFF_FFFF -1 n/a
- (YHigh:YLow) - $0000_0000:0000_0001 - 1 n/a
---------------------- --------------- --------
= $FFFF_FFFF:FFFF_FFFE = -2 Z=0, C=1
Page 278 · Propeller Manual v1.1