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 2004 Microchip Technology Inc. DS00914A-page 1

AN914

INTRODUCTION

Dynamic memory allocation is a nice functionality that

is provided with virtually all PC-based compilers. How-

ever, not all microcontroller compilers have such capa-

bility, most likely due to the lack of a sophisticated

operating system with memory management. Although

most applications are static in nature, there are cases

where a need for dynamic allocation of memory

resources exists. Examples include any number of net-

work protocols that have a dynamically specified

nature. This application note presents a simple and effi-

cient method for dynamic memory allocation without

the need of an operating system.

THE MODEL

The model is based on a simple form of a linked list. A

block of memory referred to as the dynamic heap is

split into segments. Each segment has a single-byte

header that references the next segment in the list via

an offset, as well as indicating whether the segment is

allocated. Allocation is specified by a single bit.

Figure 1 shows an example. Consequently, the

reference implicitly identifies the length of the segment.

The heap is terminated with a special header that

references itself, referred to as the “tail”.

Why use single-byte headers? The segment headers

are specifically designed to be a single byte wide to

achieve excellent execution performance, reduce code

size and minimize loss of memory space to segment

control information. Essentially, one byte references are

easier and faster to manipulate than multi-byte relative

or absolute references. Plus, they do not consume as

much space. However, some fundamental limits are

imposed by this methodology. The maximum segment

size is 126 bytes, or the size of the heap, whichever is

smaller. The smallest segment size is one byte, resulting

in a maximum number of segments of one-half of the

number of bytes in the heap minus one. For example, in

a 512-byte heap, one could expect to dynamically

allocate as many as 255 single-byte segments.

FIGURE 1: SIMPLE HEAP EXAMPLE

Although this model will allow dynamic allocation down

to a single byte, doing so sacrifices performance and

memory. With more segments within the heap, more

time is required to attempt to allocate memory. In addi-

tion, every segment requires a header byte; therefore,

a large number of smaller segments require more

memory than a small number of large segments. In the

255-segment example mentioned previously, 50% of

the heap is lost to segment header information.

There is also one other potential problem, especially

with smaller segments: memory fragmentation. Frag-

mentation could ultimately doom an application by

reducing the largest allocatable block of memory. Thus,

dynamic allocation should be restricted to larger blocks

to maintain efficiency and effective use of the heap.

Applications that are likely to encounter fragmentation

issues should provide a method to handle allocation

failures. The implementation depends on the complex-

ity of the application. For some applications, a system

Reset may be sufficient. For applications with more

advanced memory requirements, it may be necessary

to provide allocation management functions. An exam-

ple might be to force non-critical tasks to give up their

memory allocations as needed, then re-allocate

memory to them as required.

Author: Ross M. Fosler

Microchip Technology Incorporated Allocation Bit

Memory Heap

Segment

Length/Reference

Segment 1

Segment x

Tail

Dynamic Memory Allocation for the MPLAB® C18 C Compiler

AN914

DS00914A-page 2  2004 Microchip Technology Inc.

SUPPORTING FUNCTIONS

There are three functions that manage the heap:

•SRAMAlloc: Allocate memory

•SRAMFree: Free previously allocated memory

•SRAMInitHeap: Initialize the dynamic heap

SRAMAlloc

unsigned char * NEAR SRAMAlloc(NEAR unsigned

char nBytes)

SRAMAlloc is used to allocate a segment of memory

within the heap. When it is called, a new segment is

created in the heap. Essentially, larger non-allocated

segments are split to achieve the requested segment

size. If there are a number of smaller non-allocated

segments, they will be merged together to create a

single larger segment. If a segment of sufficient size

cannot be allocated, then an error is returned to the

calling application; otherwise, a 16-bit pointer to the

segment is returned, which is the next address after the

stored segment header. Figure 2 outlines the basic pro-

gram flow. The application must remember the pointer

to successfully free the memory.

SRAMFree

void SRAMFree(unsigned char * NEAR pSRAM)

This function is used to free a previously allocated

memory segment. It allows future calls to SRAMAlloc

to merge or split this segment as necessary. The

pointer returned from allocation must be passed to

successfully free the block of memory.

SRAMInitHeap

void SRAMInitHeap(void)

This function must be called at least one time to initial-

ize the heap with the minimum number of segment

headers and the tail. This function could also be called

to initialize the heap. The minimum number is always

the value of (MAX_HEAP_SIZE/126), rounded up for

any remainder. For example, a 256-byte heap will be

initialized with three segments.

FIGURE 2: SEGMENT ALLOCATION FLOWCHART

Start

Scan for Next

Non-allocated

Segment

Segment too

large?

Found tail?

Segment too

small?

allocated?

Merge Adjacent

Segments

Split Segment

Return Pointer

Return NULL

Finish

Yes

segment

 2004 Microchip Technology Inc. DS00914A-page 3

AN914

SETTING UP

Compile Time Options

There are only two compile time options to be set:

•NEAR_MODEL: This specifies whether the code

uses access registers or normal data space for

general processing. There is some small

performance improvement using access memory.

•MAX_HEAP_SIZE: This specifies the size of the

dynamic heap. This value should correlate with

the section size specified in the linker script.

The Linker Script

The source code reserves a block of memory specified

by a section in the linker file named

_SRAM_ALLOC_HEAP. Refer to the attached linker

script in Appendix D: “Linker Script” for an example.

PERFORMANCE

The performance of dynamic allocation varies signifi-

cantly depending on the build options, the number of

segments in the heap, the positions and sizes of the

segments and the size of the heap. In the example

code, with the build options selected in the example

project, allocation can occur in as little as 100 instruc-

tion cycles. In other basic tests, with 4 to 5 segments

previously allocated, allocation can occur in as much

as 450 instruction cycles.

Freeing allocated segments is relatively fixed

compared to allocation. In the example code, with the

build options selected in the example project, freeing

allocated memory only requires 18 instruction cycles.

MEMORY USAGE

The memory usage varies depending on the build

options, the number of segments in the heap, the posi-

tions and sizes of the segments and the size of the

heap. In the example code presented here and with the

build options selected in the example project, only

452 bytes of program memory and 20 bytes of data

memory are used. In addition, another 512 bytes of

data memory are reserved for the dynamic heap. Note

that the heap size can be increased or decreased to

meet the needs of the application.

Produktspezifikationen

Marke:	Microchip
Kategorie:	Nicht kategorisiert
Modell:	PIC18F2520

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