70592C Datasheet, PDF (35/314 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJXXXGPX06A/X08A/X10A
4.2 Data Address Space
The PIC24HJXXXGPX06A/X08A/X10A CPU has a
separate 16-bit wide data memory space. The data
space is accessed using separate Address Generation
Units (AGUs) for read and write operations. Data mem-
ory maps of devices with different RAM sizes are
shown in Figure 4-3 and Figure 4-4.
All Effective Addresses (EAs) in the data memory space
are 16 bits wide and point to bytes within the data space.
This arrangement gives a data space address range of
64 Kbytes or 32K words. The lower half of the data
memory space (that is, when EA<15> = 0) is used for
implemented memory addresses, while the upper half
(EA<15> = 1) is reserved for the Program Space
Visibility area (see Section 4.4.3 “Reading Data from
Program Memory Using Program Space Visibility”).
PIC24HJXXXGPX06A/X08A/X10A devices implement
up to 16 Kbytes of data memory. Should an EA point to
a location outside of this area, an all-zero word or byte
will be returned.
4.2.1 DATA SPACE WIDTH
The data memory space is organized in byte address-
able, 16-bit wide blocks. Data is aligned in data
memory and registers as 16-bit words, but all data
space EAs resolve to bytes. The Least Significant
Bytes of each word have even addresses, while the
Most Significant Bytes have odd addresses.
4.2.2
DATA MEMORY ORGANIZATION
AND ALIGNMENT
To maintain backward compatibility with PIC^® MCU
devices and improve data space memory usage
efficiency, the PIC24HJXXXGPX06A/X08A/X10A
instruction set supports both word and byte operations.
As a consequence of byte accessibility, all effective
address calculations are internally scaled to step
through word-aligned memory. For example, the core
recognizes that Post-Modified Register Indirect
Addressing mode [Ws++] will result in a value of Ws +
1 for byte operations and Ws + 2 for word operations.
Data byte reads will read the complete word that
contains the byte, using the Least Significant bit (LSb)
of any EA to determine which byte to select. The
selected byte is placed onto the Least Significant Byte
(LSB) of the data path. That is, data memory and reg-
isters are organized as two parallel byte-wide entities
with shared (word) address decode but separate write
lines. Data byte writes only write to the corresponding
side of the array or register which matches the byte
address.
All word accesses must be aligned to an even address.
Misaligned word data fetches are not supported, so
care must be taken when mixing byte and word opera-
tions, or translating from 8-bit MCU code. If a mis-
aligned read or write is attempted, an address error
trap is generated. If the error occurred on a read, the
instruction underway is completed; if it occurred on a
write, the instruction will be executed but the write does
not occur. In either case, a trap is then executed, allow-
ing the system and/or user to examine the machine
state prior to execution of the address Fault.
All byte loads into any W register are loaded into the
Least Significant Byte. The Most Significant Byte
(MSB) is not modified.
A sign-extend instruction (SE) is provided to allow
users to translate 8-bit signed data to 16-bit signed
values. Alternatively, for 16-bit unsigned data, users
can clear the Most Significant Byte of any W register by
executing a zero-extend (ZE) instruction on the
appropriate address.
4.2.3 SFR SPACE
The first 2 Kbytes of the Near Data Space, from 0x0000
to 0x07FF, is primarily occupied by Special Function
Registers (SFRs). These are used by the
PIC24HJXXXGPX06A/X08A/X10A core and peripheral
modules for controlling the operation of the device.
SFRs are distributed among the modules that they
control, and are generally grouped together by module.
Much of the SFR space contains unused addresses;
these are read as ‘0’. A complete listing of implemented
SFRs, including their addresses, is shown in Table 4-1
through Table 4-33.
Note:
The actual set of peripheral features and
interrupts varies by the device. Please
refer to the corresponding device tables
and pinout diagrams for device-specific
information.
4.2.4 NEAR DATA SPACE
The 8-Kbyte area between 0x0000 and 0x1FFF is
referred to as the Near Data Space. Locations in this
space are directly addressable via a 13-bit absolute
address field within all memory direct instructions.
Additionally, the whole data space is addressable using
MOV instructions, which support Memory Direct
Addressing mode with a 16-bit address field, or by
using Indirect Addressing mode using a working
register as an Address Pointer.
© 2011 Microchip Technology Inc.
DS70592C-page 35