Datasheet

MCP19111
DS22331A-page 68  2013 Microchip Technology Inc.
11.1.1 READING PROGRAM MEMORY AS
DATA
There are two methods of accessing constants in pro-
gram memory. The first method is to use tables of
RETLW instructions. The second method is to set a
Files Select Register (FSR) to point to the program
memory.
11.1.1.1 RETLW Instruction
The RETLW instruction can be used to provide access
to tables of constants. The recommended way to create
such a table is shown in Example 11-1.
EXAMPLE 11-1: RETLW INSTRUCTION
11.1.1.2 Indirect Read with
Files Select
Register (
FSR)
The program memory can be accessed as data by
setting bit 7 of the FSRxH register and reading the
matching INDFx register. The MOVIW instruction will
place the lower 8 bits of the addressed word in the
W register. Writes to the program memory cannot be
performed via the INDF registers. Instructions that
access the program memory via the FSR require one
extra instruction cycle to complete. Example 11-2
demonstrates accessing the program memory via an
FSR.
The HIGH directive will set bit<7> if a label points to a
location in program memory.
EXAMPLE 11-2: ACCESSING PROGRAM
MEMORY VIA FSR
11.2 Data Memory Organization
The data memory (see Figure 11-1) is partitioned into
four banks, which contain the General Purpose
Registers (GPR) and the Special Function Registers
(SFR). The Special Function Registers are located in
the first 32 locations of each bank. Register locations
20h-7Fh in Bank 0, A0h-EFh in Bank 1 and 120h-16Fh
in Bank 2 are General Purpose Registers,
implemented as static RAM. All other RAM is
unimplemented and returns ‘0’ when read. The
RP<1:0> bits of the STATUS register are the bank
select bits.
To move values from one register to another register,
the value must pass throught the W register. This
means that for all register-to-register moves, two
instruction cycles are required.
The STATUS register, shown in Register 11-1,
contains:
• the arithmetic status of the ALU
• the Reset status
• the bank select bits for data memory (RAM)
The STATUS register can be the destination for any
instruction, like any other register. If the STATUS
register is the destination for an instruction that affects
the Z, DC or C bits, then the write to these three bits is
disabled. These bits are set or cleared according to the
device logic. Furthermore, the TO
and PD bits are not
writable. Therefore, the result of an instruction with the
STATUS register as destination may be different than
intended.
constants
RETLW DATA0 ;Index0 data
RETLW DATA1 ;Index1 data
RETLW DATA2
RETLW DATA3
my_function
;… LOTS OF CODE…
MOVLW DATA_INDEX
call constants
;… THE CONSTANT IS IN W
constants
RETLW DATA0 ;Index0 data
RETLW DATA1 ;Index1 data
RETLW DATA2
RETLW DATA3
my_function
;… LOTS OF CODE…
MOVLW LOW constants
MOVWF FSR1L
MOVLW HIGH constants
MOVWF FSR1H
MOVIW 0[FSR1]
;THE PROGRAM MEMORY IS IN W
RP1 RP0
00-> Bank 0 is selected
01-> Bank 1 is selected
10-> Bank 2 is selected
11 -> Bank 3 is selected