Motorola DSP96002 - Manual (Page 257)

MOTOROLA DSP96002 USER’S MANUAL A - 69
CMPG Graphics Compare with Trivial CMPG
Accept/Reject Flags
Operation:
S2.L - S1.L (parallel data bus move)
Assembler Syntax:
CMPG S1,S2
(move syntax - see the MOVE instruction de-
scription.)
Description:
Subtract the low portion of the two operands as specified in the operation column above. No result is
stored; however, the condition codes are affected as described below.
CMPG and CMP differ primarily in the definition of the CCR condition code bits LR and R. These differenc-
es are particularly useful in performing clipping operations in graphics applications. In the code segment,
the CMP instruction tests the first point of a line, X0, against X
min
and sets LR accordingly; the FCMPG
instruction tests the second point of a line, X1, against X
min
and sets
–
R depending on the condition of LR.
Note that the line segment will be trivially accepted if A is set (and R=1), whereas the line will be trivially
rejected if
–
R is cleared (and A=0). This choice of accept/reject conditions was selected to permit the CCR
to be initialized by a single ORI instruction.
ORI #$E0,CCR ;SET A,
–
R, LR – i. e.,
;assume line is initially
;accepted and not rejected.
MOVE X:(R0)+N0,D0.L Y:(R4)+,D1.S ;get X0, X
min
CMP D1, D0 X:(R0)-N0, D0.L ;X0-X
min
, get X1
CMPG D1, D0 ;X1=X
min
Input Operand(s) Precision: 32-bit 2’s complement integer.
Output Operand Precision: n.a.
CCR Condition Codes:
C - Set if result is negative without overflow. Set if result is positive with overflow.
Cleared otherwise.
V - Set if result overflows. Cleared otherwise.
Z - Set if result is zero. Cleared otherwise.
N - Set if result is negative. Cleared otherwise.
I - Not affected.
LR - Always set (initialize for the next CMP, CMPG combination; see the example for
the FCMPG instruction.
–
R - Cleared if LR was set and result is negative without overflow. Cleared if LR was set
and result is positive with overflow. Not affected otherwise. See the example for
the FCMPG instruction.