Motorola DSP56301 user manual Sixteen-Bit Compatibility Mode

Models: DSP56301

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Central Processor Unit (CPU) Registers

Table 4-3.Status Register Bit Definitions (Continued)

Bit Number

Bit Name

Reset Value

 

Description

 

 

 

 

 

 

 

 

14

DM

0

Double-Precision Multiply Mode

 

 

 

Enables four multiply/MAC operations to implement a double-precision

 

 

 

algorithm that multiplies two 48-bit operands with a 96-bit result. Clearing

 

 

 

the DM bit disables the mode.

 

 

 

Note:

The Double-Precision Multiply mode is supported to maintain

 

 

 

 

object code compatibility with devices in the DSP56000 family. For

 

 

 

 

a more efficient way of executing double precision multiply, refer

 

 

 

 

to the chapter on the Data Arithmetic Logic Unit in the DSP56300

 

 

 

 

Family Manual.

 

 

 

In Double-Precision Multiply mode, the behavior of the four specific

 

 

 

operations listed in the double-precision algorithm is modified. Therefore,

 

 

 

do not use these operations (with those specific register combinations) in

 

 

 

Double-Precision Multiply mode for any purpose other than the double

 

 

 

precision multiply algorithm. All other Data ALU operations (or the four

 

 

 

listed operations, but with other register combinations) can be used.

 

 

 

The double-precision multiply algorithm uses the Y0 Register at all stages.

 

 

 

Therefore, do not change Y0 when running the double-precision multiply

 

 

 

algorithm. If the Data ALU must be used in an interrupt service routine, Y0

 

 

 

should be saved with other Data ALU registers to be used and restored

 

 

 

before the interrupt routine terminates.

 

 

 

 

13

SC

0

Sixteen-Bit Compatibility Mode

 

 

 

Affects addressing functionality, enabling full compatibility with object code

 

 

 

written for the DSP56000 family. When SC is set, MOVE operations to/from

 

 

 

any of the following PCU registers clear the eight MSBs of the destination:

 

 

 

LA, LC, SP, SSL, SSH, EP, SZ, VBA and SC. If the source is either the SR

 

 

 

or OMR, then the eight MSBs of the destination are also cleared. If the

 

 

 

destination is either the SR or OMR, then the eight MSBs of the destination

 

 

 

are left unchanged. To change the value of one of the eight MSBs of the SR

 

 

 

or OMR, clear SC.

 

 

 

SC also affects the contents of the Loop Counter Register. If SC is cleared

 

 

 

(normal operation), then a loop count value of zero causes the loop body to

 

 

 

be skipped, and a loop count value of $FFFFFF causes the loop to execute

 

 

 

the maximum number of 224 – 1 times. If the SC bit is set, a loop count

 

 

 

value of zero causes the loop to execute 216 times, and a loop count value

 

 

 

of $FFFFFF causes the loop to execute 216 – 1 times.

 

 

 

Note:

Due to pipelining, a change in the SC bit takes effect only after

 

 

 

 

three instruction cycles. Insert three NOP instructions after the

 

 

 

 

instruction that changes the value of this bit to ensure proper

 

 

 

 

operation.

 

 

 

 

12

 

0

Reserved. Write to 0 for future compatibility.

 

 

 

 

 

Core Configuration

4-9

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Motorola DSP56301 user manual Sixteen-Bit Compatibility Mode