© 2000 Scenix Semiconductor, Inc. All rights reserved. - 31 - www.scenix.com SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75 15.4    RAM Addressing Direct Addressing
The  FSR  register  must  initialized  with  an  appropriate
value in order to address the desired RAM register. The
following  table  and  code  example  show  how  to  directly
access the banked registers. Indirect Addressing
To  access  any  register  via  indirect  addressing,  simply
move the eight-bit address of the desired register into the
FSR and use INDF as the operand. The example below
shows how to clear all RAM locations from 10h to 1Fh in
all eight banks: 15.5    The Bank Instruction Often it is desirable to set the bank select bits of the FSR
register  in  one  instruction  cycle.  The  Bank  instruction
provides  this  capability.  This  instruction  sets  the  upper
bits of the FSR to point to a specific RAM bank without
affecting the other FSR bits.
Example: 15.6    Bit Manipulation The instruction set contains instructions to set, reset, and
test individual bits in data memory. The device is capable
of bit addressing anywhere in data memory. 15.7    Input/Output Operation The device contains three registers associated with each
I/O port. The first register (Data Direction Register), con-
figures each port pin as a Hi-Z input or output. The sec-
ond  register  (TTL/CMOS  Register),  selects  the  desired
input level for the input. The third register (Pull-Up Regis-
ter), enables a weak pull-up resistor on the pin configured
as a input. In addition to using the associated port regis-
ters, appropriate values must be written into the MODE
register to configure the I/O ports.   
When two successive read-modify-write instructions are
used on the same I/O port with a very high clock rate, the
“write”   part   of   one   instruction   might   not   occur   soon
enough before the “read” part of the very next instruction,
resulting in getting “old” data for the second instruction.
To ensure predictable results, avoid using two successive
read-modify-write instructions that access the same port
data register if the clock rate is high. 15.8    Increment/Decrement The bank of 31 registers serves as a set of accumulators.
The instruction set contains instructions to increment and
decrement the register file. The device also includes both
INCSZ  fr  (increment  file  register  and  skip  if  zero)  and
DECSZ   fr   (decrement   file   register   and   skip   if   zero)
instructions. 15.9    Loop Counting and Data Pointing Testing The   device   has   specific   instructions   to   facilitate   loop
counting. The DECSZ fr (decrement file register and skip
if  zero) tests  any one of  the  file  registers  and  skips the
next instruction (which can be a branch back to loop)  if
the result is zero. 15.10    Branch and Loop Call Instructions The device contains an 8-level hardware stack where the
return  address  is  stored  with  a  subroutine  call.  Multiple
stack levels allow subroutine nesting. The instruction set
supports absolute address branching.
15.10.1  Jump Operation
When a JMP instruction is executed, the lower nine bits
of the program counter is loaded with the address of the
specified   label.   The   upper   two   bits   of   the   program
counter  are  loaded  with  the  page  select  bits,  PA1:PA0,
contained in the STATUS register. Therefore, care must
be exercised to ensure the page select bits are pointing
to the correct page before the jump occurs. Bank FSR Value 0 010h 1 030h 2 050h 3 070h 4 090h 5 0B0h 6 0D0h 7 0F0h mov FSR,#$070 ;Select RAM Bank 3 clr $010 ;Clear register 10h on
;Bank 3 mov FSR,#$D0 ;Select RAM Bank 6 clr $010 ;Clear register 10h on
;Bank 6 clr FSR ;clear FSR to 00h (at address
;04h) :loop  setb   SFR.4 ;set bit 4: address 10h-1Fh,
;30-3Fh, etc clr INDF ;clear register pointed to by
;FSR incsz  FSR ;increment FSR and test, skip
;jmp if 00h jmp :loop ;jump back and clear next
;register bank    $F0 ;Select Bank 7 in FSR inc $1F ;increment file register
;1Fh in Bank 7 STATUS<6:5> JMP LABEL PC<10:9> PC<8:0>