© 2000 Scenix Semiconductor, Inc. All rights reserved. 31 SX User’s Manual Rev. 3.1 www.scenix.com Chapter 2 Architecture 2.4.1 W (Working Register) The W register is the main working register used by many instructions as the source or destination of
the operation. It is often used as a temporary storage area for intermediate operations. For example, to
add the contents of two file registers, you must first move the contents of one file register to W and
then execute an “add” instruction to perform an addition between W and the other file register. For  SX18/20/28AC  and  SX18/20/28AC75  devices,  in  the  default  device  configuration,  W  is  not
memory-mapped and can only be accessed by instructions that work specifically with W as the source
or destination. However, you can optionally make the W available as a memory-mapped register at
address 01h. To  do this,  first program the OPTIONX bit to 0  in the FUSEX word in the  program
memory. Then have your program clear the RTW bit in the OPTION register. If you do this, the RTCC
register normally at address 01h becomes unavailable. 2.4.2 INDF (Indirect through FSR) The INDF register location (address 00h) is used for indirect addressing. Whenever this address is
specified as the source or destination of an operation, the device uses the register pointed to by the FSR
register (address 04h). There is no actual register or data stored at address 00h. For more information on indirect addressing, see Section 2.3. 2.4.3 RTCC (Real-Time Clock/Counter) The RTCC register (address 01h) is an 8-bit Real-Time Clock/Counter used to keep track of elapsed
time or to keep a count of transitions on the RTCC input pin. The timer operating configuration is
determined by control bits in the OPTION register. To keep track of time, you configure the timer register to be incremented once per instruction cycle or
once per multiple of the instruction cycle. To count external events, you configure the timer register to
be incremented once per rising edge or falling edge on the RTCC input pin. The  program  can  read  or  write  the register  at  any time. A  rollover  from  FFh  to  00h generates  an
interrupt to the CPU if that condition is enabled as an interrupt. For more information on the operation of the timer, see Section 6.2. In the Sx18/20/28AC and Sx18/20/28AC75 devices, if you do not need to use the RTCC register, you
can optionally make the working register (W) available as a memory-mapped register at address 01h.
For details, see the description of the W register. 2.4.4 PC (Program Counter) The PC register (address 02h) contains the lower eight bits of the 11-bit or 12-bit program counter. The
program counter is a pointer register that points to the current instruction being executed in the 2,048-
word  or  4,096-word  program  memory.  During  regular  program  execution,  the  program  counter  is
incremented  automatically  once  per  instruction  cycle.  This  regular  sequence  is  altered  in  order  to
perform skips, jumps, and subroutine calls in the application program.