Motorola DSP56305 Manuel d'utilisateur télécharger pdf (Page 37)

3-8 Optimizing DSP56300/DSP56600 Applications MOTOROLA

Program Control

Using the Stack Extension

following formula, which takes into account that each increment in

SZ corresponds to two memory locations:

For example, if the memory extension space available is 1024 words,

SZ should be set to 1024/2 + 14 = 526. SZ should be set to an even

number since stack extension transfers are done in pairs.

SC is a 5-bit register that stores the number of entries in the

hardware stack. SC is related to the stack only when the stack

extension is enabled. A push increments SC by 1, until the value 14 is

reached. If SC equals 14 and a push occurs, the push is executed, and

the least recently used stack entry (2 words) is copied to the

extension, leaving SC with the same value of 14 (hardware stack-full

state). A pop decrements SC by 1, until the value 2 is reached. If SC

equals 2 and SP > SC, when a stack pop occurs, the pop is executed,

and the top entry of the extension (2 words) is copied from the

memory to the stack, replacing the entry just read, thus leaving SC

with the same value of 2 (hardware stack-empty state).

Note: In principle there is no forced connection between the values

of SP and SC.

EP holds the pointer to the data memory location where the

extension is stored. The address space (X or Y) is selected by setting

the XYS bit in the OMR. EP has no default value and should be

initialized by the user. Each push that activates the extension causes

two memory writes, after which EP is incremented by 2, since one

stack entry is composed of 2 words. Similarly, each pop that

activates the extension causes 2 memory reads, after which EP is

decremented by 2. There is no restriction on the value of EP

(internal or external memory space), however in the DSP56300

family, the user should be aware that setting EP to point to external

memory will generate external accesses with possible wait states,

depending on the external memory type. The DSP56600 family does

not support external data accesses.

When the stack extension is disabled, the stack status information

resides in Bits 4 and 5 of SP (named SE and UF, respectively).

Normally, the stack error routine should consult these bits. When

using the stack extension, however, all stack status information

resides in the OMR. The SP bits do not reflect stack status

information as they are now part of the stack pointer value. The

stack status bits are also functionality different, as summarized in

SZ available memory extension size 2⁄ 14+=

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 111 112

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Motorola DSP56305 Manuel d'utilisateur Page 37