© 1999 Scenix Semiconductor, Inc. All rights reserved. - 10 - www.scenix.com SX18AC  / SX20AC / SX28AC 4.0 SPECIAL-FUNCTION REGISTERS The CPU uses a set of special-function registers to con-
trol the operation of the device.
The CPU registers include an 8-bit working register (W),
which serves as a pseudo accumulator. It holds the sec-
ond   operand   of   an   instruction,   receives   the   literal   in
immediate  type  instructions,  and  also  can  be  program-
selected as the destination register.
A set of 31 file registers serves as the primary accumula-
tor. One of these registers holds the first operand of an
instruction and  another can be program-selected as the
destination  register.  The  first  eight  file  registers  include
the Real-Time Clock/Counter register (RTCC), the lower
eight  bits  of  the  11-bit  Program  Counter  (PC),  the  8-bit
STATUS register, three port control registers for Port A,
Port B, Port C, the 8-bit File Select Register (FSR), and
INDF used for indirect addressing.
The five low-order bits of the FSR register select one of
the 31 file registers in the indirect addressing mode. Call-
ing for the file register located at address 00h (INDF) in
any   of   the   file-oriented   instructions   selects   indirect
addressing,  which  uses  the  FSR  register.  It  should  be
noted that the file register at address 00h is not a physi-
cally implemented register. The CPU also contains an 8-
level, 11-bit hardware push/pop stack for subroutine link-
age. *In the SX18 package, Port C is not used, and address
07h is available as a general-purpose RAM location. 4.1 PC Register (02h) The PC register holds the lower eight bits of the program
counter.  It  is  accessible  at  run  time  to  perform  branch
operations. 4.2 STATUS Register (03h) The STATUS register holds the arithmetic status of the
ALU,   the   page   select   bits,   and   the   reset   state.   The
STATUS  register  is  accessible  during  run  time,  except
that  bits  PD  and  TO  are  read-only.  It  is  recommended
that  only  SETB  and  CLRB  instructions  be  used  on  this
register.  Care  should  be  exercised  when  writing  to  the STATUS  register  as  the  ALU  status  bits  are  updated
upon completion of the write operation, possibly leaving
the  STATUS  register  with  a  result  that  is  different  than
intended. Table  4-1.  Special-Function Registers Addr Name Function 00h INDF Used for indirect addressing 01h RTCC Real Time Clock/Counter 02h PC Program Counter (low byte) 03h STATUS Holds Status bits of ALU 04h FSR File Select Register 05h RA Port RA Control register 06h RB Port RB Control register 07h RC* Port RC Control register PA2 PA1 PA0 TO PD Z DC C Bit 7 Bit 0 Bit 7-5:  Page select bits PA2:PA0 000 = Page 0 (000h – 01FFh)
001 = Page 1 (200h – 03FFh)
010 = Page 2 (400h – 05FFh)
011 = Page 3 (600h – 07FFh) Bit 4: Time Out bit, TO 1 = Set to 1 after power up and upon exe-
cution of CLRWDT or SLEEP instructions
0 = A watchdog time-out occurred Bit 3: Power Down bit, PD 1= Set to a 1 after power up and upon ex-
ecution of the CLRWDT instruction
0 = Cleared to a ‘0’ upon execution of
SLEEP instruction Bit 2: Zero bit, Z 1 = Result of math operation is zero
0 = Result of math operation is non-zero Bit 1: Digit Carry bit, DC After Addition:
1 = A carry from bit 3 occurred
0 = No carry from bit 3 occurred
After Subtraction:
1 = No borrow from bit 3 occurred
0 = A borrow from bit 3 occurred Bit 0: Carry bit, C After Addition:
1 = A carry from bit 7 of the result occured
0 = No carry from bit 7 of the result oc-
cured
After Substraction:
1 = No borrow from bit 7 of the result oc-
cured
0 = A borrow from bit 7 of the result oc-
cured
Rotate (RR or RL) Instructions:
The carry bit is loaded with the low or high
order bit, respectively. When CF bit is
cleared, Carry bit works as input for ADD
and SUB instructions.