© 2000 Scenix Semiconductor, Inc. All rights reserved. - 30 - www.scenix.com SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75 15.0    INSTRUCTION SET As  mentioned  earlier,  the  SX  family  of  devices  uses  a
modified   Harvard    architecture    with    memory-mapped
input/output. The device also has a RISC type architec-
ture  in  that  there  are  43  single-word  basic  instructions.
The instruction set contains byte-oriented file register, bit-
oriented file register, and literal/control instructions.
Working  register  W  is  one  of  the  CPU  registers,  which
serves as a pseudo accumulator. It is a pseudo accumu-
lator   in   a   sense   that   it   holds   the   second   operand,
receives the literal in the immediate type instructions, and
also can be program-selected as the destination register.
The  bank  of  31  file  registers  can  also  serve  as  the  pri-
mary accumulators, but they represent the first operand
and  may  be  program-selected  as  the  destination  regis-
ters. 15.1    Instruction Set Features 1.  All single-word (12-bit) instructions for compact code       efficiency.
2.  All instructions are single cycle except the jump type in-       structions (JMP, CALL) and failed test instructions
      (DECSZ fr, INCSZ fr, SB bit, SNB bit), which are two-
      cycle.
3.  A set of File registers can be addressed directly or indi-       rectly, and serve as accumulators to provide first oper-
      and; W register provides the second operand.
4.  Many instructions include a destination bit which se-       lects either the register file or the accumulator as the
      destination for the result.
5.  Bit manipulation instructions (Set, Clear, Test and Skip       if Set, Test and Skip if Clear).
6.  STATUS Word register memory-mapped as a register       file, allowing testing of status bits (carry, digit carry, ze-
      ro, power down, and timeout).
7.  Program Counter (PC) memory-mapped as register file       allows W to be used as offset register for indirect ad-
      dressing of program memory.
8.  Indirect addressing data pointer FSR (file select regis-       ter) memory-mapped as a register file.
9.  IREAD instruction allows reading the instruction from       the program memory addressed by W and upper four
      bits of MODE register.
10.Eight-level, 11-bit push/pop hardware stack for sub-       routine linkage using the Call and Return instructions.
11.Six addressing modes provide great flexibility. 15.2    Instruction Execution An  instruction  goes  through  a  four-stage  pipeline  to  be
executed  (Figure 15-1).  The  first  instruction  is  fetched
from the program memory on the first clock cycle. On the
second  clock  cycle,  the  first  instruction  is  decoded  and
the second instruction is fetched. On the third clock cycle,
the first instruction is executed, the second instruction is
decoded,  and  the  third  instruction  is  fetched.  On  the
fourth clock cycle, the first instruction’s results are written
to its destination, the second instruction is executed, the
third instruction is decoded, and the fourth instruction is fetched.  Once  the  pipeline  is  full,  instructions  are  exe-
cuted at the rate of one per clock cycle.
Instructions  that  directly  affect  the  contents  of  the  pro-
gram counter (such as jumps and calls) require that the
pipeline be cleared and subsequently refilled. Therefore,
these instruction take more than one clock cycle.
The instruction execution time is derived by dividing the
oscillator  frequency  by  either  one  (turbo  mode)  or  four
(non-turbo   mode).   The   divide-by   factor   is   selected
through the FUSE Word register. 15.3    Addressing Modes The device support the following addressing modes:       Data Direct
      Data Indirect
      Immediate
      Program Direct
      Program Indirect
      Relative
Both direct and indirect addressing modes are available.
The INDF register, though physically not implemented, is
used in conjunction with the indirect data pointer (FSR) to
perform indirect addressing. An instruction using INDF as
its  operand  field  actually  performs  the  operation  on  the
register  pointed  by  the  contents  of  the  FSR.  Conse-
quently, processing two multiple-byte operands requires
alternate loading of the operand addresses into the FSR
pointer as the multiple byte data fields are processed.
Examples:
Direct addressing: Indirect Addressing: Figure 15-1. Pipeline and Clock Scheme mov RA,#01 ;move “1” to RA mov FSR,#RA ;FSR = address of RA mov INDF,#$01 ;move “1” to RA Fetch Decode Execute Write Clock
Cycle 1 Clock
Cycle 2 Clock
Cycle 3 Clock
Cycle 4