Data Memory Spaces; X Data Memory Space - Motorola DSP56305 User Manual

3.1.2

Data Memory Spaces

Data memory space is divided into X data memory and Y data memory to match the
natural partitioning of DSP algorithms. The data memory partitioning allows the
DSP56305 to feed two operands to the Data ALU simultaneously, enabling it to perform
a multiply-accumulate operation in one clock cycle.
X and Y data memory space are similar in structure and functionality. They each contain:
• Internal data RAM memory
• (Optionally) Off-chip memory expansion (up to 16 M in the 24-bit Address mode
and 64 K in the 16-bit Address mode).
There are several differences between the two memory spaces.
1. The X data RAM size is 3.75 K by default, while Y data RAM size is 2 K.
2. The upper 128 words of each space are reserved for different uses. The upper 128
words of X data memory are reserved for internal I/O. The upper 64 words of Y
data memory may be used for more internal and for external I/O space (for
further information, see Section 3.1.2.1 and Section 3.1.2.2).
3. The Y memory space contains a ROM, which may be used for customer-supplied
code.
For further information on supplying code for a customized DSP56305 ROM, please
contact your Motorola regional sales office.
3.1.2.1

X Data Memory Space

The on-chip X data RAM is 24-bit wide, high-speed, internal Static RAM occupying by
default the lowest 3840 (3.75 K) locations ($000000–$000EFF) in X memory space. It is
organized in 15 banks with 256 locations each.
When the Memory Switch (MS) bit (OMR Bit 7) is set, the 1024 (1 K, or four banks)
uppermost locations ($000B00–$00000EFF) of X data RAM are switched to Program
RAM. There is then 2.75 K of X data RAM, and the highest internal X data RAM location
is then $000AFF. The default value of the MS bit is cleared.
The on-chip peripheral registers and some of the DSP56305 core registers occupy the top
128 locations of X data memory ($FFFF80–$FFFFFF in the 24-bit Address mode or
$FF80–$FFFF in the 16-bit Address mode). This area is called X-I/O space, and it can be
accessed by MOVE and MOVEP instructions and by bit-oriented instructions (BCHG,
BCLR, BSET, BTST, BRCLR, BRSET, BSCLR, BSSET, JCLR, JSET, JSCLR, and JSSET). For
a listing of the contents of this area, see the Programming Sheets in Appendix D.
The X memory space at locations $FF0000–$FFEFFF is reserved and should not be
accessed.
MOTOROLA DSP56305 User's Manual
Memory Configuration
Memory Spaces
3-5