Answers Database

XBLOX: ACCUM - effects of LOAD on C_OUT and OVFL


Record #2726

Product Family:  Software

Product Line:  FPGA Core

Problem Title:
XBLOX: ACCUM - effects of LOAD on C_OUT and OVFL


Problem Description:
Keywords: XBLOX, ACCUM, LOAD, C_OUT, OVFL

Urgency: standard

General Description:
What is the effect on the C_OUT and OVFL outputs of asserting LOAD on the
XBLOX ACCUM macro?


Solution 1:

The synchronous LOAD function of the XBLOX ACCUM macro is implemented via a
2-to-1 multiplexer.  The LOAD signal acts as the select line on the mux.
When LOAD is high, the mux passes the B input to the ACCUM register; when
LOAD is low, the mux passes the ADD/SUB value to the ACCUM register.

LOAD has no direct effect on the C_OUT and OVFL outputs.  During a LOAD
operation, the C_OUT and OVFL outputs will continue to reflect the carry
out and overflow status of the ADD/SUB logic.  However, since LOAD affects
Q_OUT, which drives the A input of the ADD/SUB logic, it does affect C_OUT
and OVFL indirectly.



End of Record #2726