Altera Corporation
2–15
May 2007
Stratix II Device Handbook, Volume 1
Stratix II Architecture
Figure 2–11. ALM in Arithmetic Mode
While operating in arithmetic mode, the ALM can support simultaneous
use of the adder's carry output along with combinational logic outputs. In
this operation, the adder output is ignored. This usage of the adder with
the combinational logic output provides resource savings of up to 50% for
functions that can use this ability. An example of such functionality is a
conditional operation, such as the one shown in
Figure 2–12. The
equation for this example is:
R = (X < Y) ? Y : X
To implement this function, the adder is used to subtract ‘Y’ from ‘X.’ If
‘X’ is less than ‘Y,’ the carry_out signal is ‘1.’ The carry_out signal is
fed to an adder where it drives out to the LAB local interconnect. It then
feeds to the LAB-wide syncload signal. When asserted, syncload
selects the syncdata input. In this case, the data ‘Y’ drives the
syncdata
inputs to the registers. If ‘X’ is greater than or equal to ‘Y,’ the
syncload
signal is de-asserted and ‘X’ drives the data port of the
registers.
dataf0
datae0
carry_in
carry_out
dataa
datab
datac
datad
datae1
dataf1
DQ
To general or
local routing
To general or
local routing
reg0
reg1
To general or
local routing
To general or
local routing
4-Input
LUT
4-Input
LUT
4-Input
LUT
4-Input
LUT
adder1
adder0
