Siemens Simatic S7 Series System Manual page 64

PLC concepts
4.2 Accessing data
HC (high-speed counter)
The high-speed counters count high-speed events independent of the CPU scan. High-
speed counters have a signed, 32-bit integer counting value (or current value). To access
the count value for the high-speed counter, you specify the address of the high-speed
counter, using the memory type (HC) and the counter number. The current value of the high-
speed counter is a read-only value and can be addressed only as a double word (32 bits).
Table 4- 10
High-speed counter
AC (accumulators)
The accumulators are read/write devices that can be used like memory. For example, you
can use accumulators to pass parameters to and from subroutines and to store intermediate
values used in a calculation. The CPU provides four 32-bit accumulators (AC0, AC1, AC2,
and AC3). You can access the data in the accumulators as bytes, words, or double words.
The size of the data being accessed is determined by the instruction that is used to access
the accumulator. As shown in the following figure, you use the least significant 8 or 16 bits of
the value that is stored in the accumulator to access the accumulator as bytes or words. To
access the accumulator as a double word, you use all 32 bits.
For information about how to use the accumulators within interrupt subroutines, refer to the
Interrupt instructions (Page 282).
Table 4- 11
Accumulator
64
Absolute addressing of HC memory
Absolute addressing of AC memory
HC [high-speed counter number]
[accumulator number]
AC
System Manual, 09/2015, A5E03822230-AC
HC1
AC0
S7-200 SMART