© 1999 Scenix Semiconductor, Inc. All rights reserved. - 14 - www.scenix.com SX18AC  / SX20AC / SX28AC 6.0    MEMORY ORGANIZATION
6.1    Program Memory The  program  memory  is  organized  as  2K,  12-bit  wide
words.   The   program   memory   words   are   addressed
sequentially  by  a  binary  program  counter.  The  program
counter starts at zero. If there is no branch operation, it
will  increment  to  the  maximum  value  possible  for  the
device and roll over and begin again.
Internally,  the  program  memory  has  a  semi-transparent
page  structure.  A page  is  composed  of  512  contiguous
program memory words. The lower nine bits of the pro-
gram counter are zeros at the first address of a page and
ones at the last address of a page. This page structure
has  no  effect  on   the  program  counter.  The   program
counter  will  freely  increment  through  the  page  bound-
aries.
6.1.1  Program Counter
The program counter contains the 11-bit address of the
instruction to be executed. The lower eight bits of the pro-
gram counter are contained in the PC register (02h) while
the upper bits come from the upper three bits of the STA-
TUS register (PA0, PA1, PA2). This is necessary to cause
jumps   and   subroutine   calls   across   program   memory
page boundaries. Prior to the execution of a branch oper-
ation,  the  user  program must  initialize  the  upper  bits of
the  STATUS  register  to  cause  a  branch  to  the  desired
page. An alternative method is to use the PAGE instruc-
tion,  which  automatically  causes  branch  to  the  desired
page, based on the value specified in the operand field.
Upon reset, the program counter is initialized with 07FFh.
6.1.2  Subroutine Stack
The subroutine stack consists of eight 11-bit save regis-
ters. A physical transfer of register contents from the pro-
gram counter  to  the  stack  or  vice  versa,  and  within  the
stack, occurs on all operations affecting the stack, prima-
rily calls and returns. The stack is physically and logically
separate  from  data  RAM.  The  program  cannot  read  or
write the stack. 6.2    Data Memory The  data  memory  consists  of  136  bytes  of  RAM,  orga-
nized as eight banks of 16 registers plus eight registers
which  are  not  banked.  Both  banked  and  non-banked
memory locations can be addressed directly or indirectly
using  the  FSR  (File  Select  Register).  The  special-func-
tion registers are mapped into the data memory.
6.2.1  File Select Register (04h)
Instructions  that  specify  a  register  as  the  operand  can
only  express  five  bits  of  register  address.  This  means
that only registers 00h to 1Fh can be accessed. The File
Select Register (FSR) provides the ability to access reg-
isters beyond 1Fh.
Figure 6-1 shows how FSR can be used to address RAM
locations. The three high-order bits of FSR select one of
eight  SRAM  banks  to  be  accessed.  The  five  low-order
bits select one of 32 SRAM locations within the selected
bank.  For  the  lower  16  addresses,  Bank  0  is  always
accessed, irrespective of the three high-order bits. Thus,
RAM register addresses 00h through 0Fh are “global” in
that they can always be accessed, regardless of the con-
tents of the FSR.
The entire data memory (including the dedicated-function
registers) consists of the lower 16  bytes of  Bank  0 and
the upper 16 bytes of Bank 0 through Bank 7, for a total
of (1+8)*16 = 144 bytes. Eight of these bytes are for the
function registers, leaving 136 general-purpose memory
locations. In the 18-pin SX packages, register RC is not
used,  which  makes  address  07h  available  as  an  addi-
tional general-purpose memory location.
Below  is  an  example  of  how  to  write  to  register  10h  in
Bank 4: mov FSR,#$90 ;Select Bank 4 by
;setting FSR<7:5> mov $10,#$64 ;load register 10h with
;the literal 64h