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X Data Memory Space

DSP56303 User’s Manual, Rev. 2

Freescale Semiconductor

3-3

3.2 X Data Memory Space

The X data memory space consists of the following:

Internal X data memory (2 K by default up to 3 K)

Internal I/O space (upper 128 locations)

Optional off-chip memory expansion (up to 64 K in 16-bit mode, or 256 K in 24-bit mode

using the 18 external address lines, or 4 M using the external address lines and the four

address attribute lines). Refer to the DSP56300 Family Manual, especially Chapter 9,

External Memory Interface (Port A), for details on using the external memory interface to

access external X data memory.

Note:

The X memory space at $FF0000–$FFEFFF is reserved and should not be accessed.

3.2.1 Internal X Data Memory

The default internal X data RAM is a 24-bit-wide, internal, static memory occupying the lowest

2 K locations ($000–$7FF) in X memory space. The internal X data RAM is organized into 8

banks with 256 locations each. Available X data memory space is increased by 1 K through

reallocation of program memory using the memory switch mode described in the next section.

3.2.2 Memory Switch Modes—X Data Memory

Memory switch mode reallocates portions of program RAM to X and Y data memory. Bit 7 in the

OMR is the MS bit that controls this function, as follows:

When the MS bit is cleared, the X data memory consists of the default 2 K × 24-bit

memory space described in the previous section. In this default mode, the lowest external

X data memory location is $800.

When the MS bit is set, a portion of the higher locations of the internal program memory is

switched to X and Y data memory. The X data memory in this mode consists of a

3 K

× 24-bit memory space. In this mode, the lowest external X data memory location is

$C00.

3.2.3 Internal I/O Space—X Data Memory

One part of the internal peripheral registers and some of the DSP56303 core registers occupy the

top 128 locations of the X data memory ($FFFF80–$FFFFFF). This area is referred to as the

internal X I/O space and it can be accessed by move, movep instructions and by bit-oriented

instructions (bchg, bclr, bset, btst, brclr, brset, bsclr, bsset, jclr, jset, jsclr, and jsset). The

contents of the internal X I/O memory space are listed in Appendix A, Bootstrap Program.

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