Motorola DSP96002 - Manual (Page 296)

A - 108 DSP96002 USER’S MANUAL MOTOROLA
FCMPG Graphics Compare FCMPG
with Trivial Accept/Reject Flags
Operation:
S2 - S1 (parallel data bus move)
Assembler Syntax:
FCMPG S1,S2 (move syntax - see the MOVE in-
struction description.)
Description:
Subtract the two operands as specified in the operation column above. No result is stored; however, the
condition codes are affected as described. This instruction differs from FSUB when S1=S2; in this case,
the result is always +0 and therefore, N is cleared. Note that this is true even if S1, S2 are infinity.
FCMPG and FCMP differ primarily in the definition of the CCR condition code bits LR and R. These differ-
ences are particularly useful in performing clipping operations in graphics applications. In the code seg-
ment, the FCMP instruction tests the first point of a line, X0, against X
min
and sets LR accordingly; the FC-
MPG 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.S Y:(R4)+,D1.S ;get X0, X
min
FCMP D1, D0 X:(R0)-N0, D0.S ;X0-X
min
, get X1
FCMPG D1, D0 ;X1=X
min
Input Operand(s) Precision: SEP Floating-Point.
Output Operand Precision: n.a.