Microchip Technology MA300015 Data Sheet

DS70150E-page 26

© 2011 Microchip Technology Inc.

3.1.2

DATA ACCESS FROM PROGRAM 
MEMORY USING PROGRAM SPACE 
VISIBILITY

The upper 32 Kbytes of data space may optionally be
mapped into any 16K word program space page. This
provides transparent access of stored constant data
from X data space, without the need to use special
instructions (i.e., TBLRDL/H, TBLWTL/H instructions). 

Program space access through the data space occurs
if the MSb of the data space EA is set and program
space visibility is enabled, by setting the PSV bit in the
Core Control register (CORCON). The functions of
CORCON are discussed in 

Data accesses to this area add an additional cycle to
the instruction being executed, since two program
memory fetches are required. 

Note that the upper half of addressable data space is
always part of the X data space. Therefore, when a
DSP operation uses program space mapping to access
this memory region, Y data space should typically con-
tain state (variable) data for DSP operations, whereas
X data space should typically contain coefficient
(constant) data.

Although each data space address, 0x8000 and higher,
maps directly into a corresponding program memory
address (see Figure 3-5), only the lower 16 bits of the
24-bit program word are used to contain the data. The
upper 8 bits should be programmed to force an illegal
instruction to maintain machine robustness. Refer
to the “16-bit MCU and DSC Programmer’s Reference
Manual” (DS70157) for details on instruction encoding.

Note that by incrementing the PC by 2 for each
program memory word, the Least Significant 15 bits of
data space addresses directly map to the Least Signif-
icant 15 bits in the corresponding program space
addresses. The remaining bits are provided by the
Program Space Visibility Page register, PSVPAG<7:0>,
as shown in 

.

For instructions that use PSV which are executed
outside a REPEAT loop:

• The following instructions will require one instruc-

tion cycle in addition to the specified execution 
time: 

- MAC class of instructions with data operand 

prefetch

- MOV instructions

- MOV.D instructions

• All other instructions will require two instruction 

cycles in addition to the specified execution time 
of the instruction.

For instructions that use PSV which are executed
inside a REPEAT loop:

• The following instances will require two instruction 

cycles in addition to the specified execution time 
of the instruction:

- Execution in the first iteration

- Execution in the last iteration

- Execution prior to exiting the loop due to an 

interrupt

- Execution upon re-entering the loop after an 

interrupt is serviced

• Any other iteration of the REPEAT loop will allow 

the instruction, accessing data using PSV, to 
execute in a single cycle.

0

8

16

PC Address

0x000000

0x000002

0x000004
0x000006

23

Program Memory
‘Phantom’ Byte
(Read as ‘0’)

TBLRDH.W

TBLRDH.B (Wn<0> = 1)

TBLRDH.B (Wn<0> = 0)

PSV access is temporarily disabled during
table reads/writes.