Memory Management¶

Comprehensive guide to memory management in Nexus embedded applications.

Overview ¶

Effective memory management is critical for embedded systems with limited RAM. This guide covers memory allocation strategies, optimization techniques, and debugging tools.

Memory Types:

Stack - Automatic variables, function calls
Heap - Dynamic allocation
Static - Global and static variables
Code - Program instructions (Flash/ROM)

Key Concepts:

Memory layout and sections
Stack and heap management
Memory pools
Memory fragmentation
Memory debugging

Memory Layout ¶

Typical ARM Cortex-M Layout ¶

┌─────────────────────┐ 0xFFFFFFFF
│   Device Registers  │
├─────────────────────┤ 0x40000000
│   External RAM      │
├─────────────────────┤ 0x20000000
│   SRAM              │
│  ┌───────────────┐  │
│  │  Heap (grows→)│  │
│  ├───────────────┤  │
│  │  Free Space   │  │
│  ├───────────────┤  │
│  │ (←grows Stack)│  │
│  ├───────────────┤  │
│  │  .bss (zero)  │  │
│  ├───────────────┤  │
│  │  .data (init) │  │
│  └───────────────┘  │
├─────────────────────┤ 0x08000000
│   Flash/ROM         │
│  ┌───────────────┐  │
│  │  .text (code) │  │
│  ├───────────────┤  │
│  │  .rodata      │  │
│  └───────────────┘  │
└─────────────────────┘ 0x00000000

Memory Sections:

.text - Program code
.rodata - Read-only data (const)
.data - Initialized global/static variables
.bss - Zero-initialized global/static variables
.heap - Dynamic allocation
.stack - Function call stack

Linker Script ¶

Example Linker Script (STM32F4):

MEMORY
{
    FLASH (rx)  : ORIGIN = 0x08000000, LENGTH = 1024K
    RAM (rwx)   : ORIGIN = 0x20000000, LENGTH = 192K
}

SECTIONS
{
    .text :
    {
        *(.isr_vector)
        *(.text*)
        *(.rodata*)
    } > FLASH

    .data :
    {
        _sdata = .;
        *(.data*)
        _edata = .;
    } > RAM AT> FLASH

    .bss :
    {
        _sbss = .;
        *(.bss*)
        *(COMMON)
        _ebss = .;
    } > RAM

    ._user_heap_stack :
    {
        . = ALIGN(8);
        PROVIDE(end = .);
        PROVIDE(_heap_start = .);
        . = . + _Min_Heap_Size;
        . = . + _Min_Stack_Size;
        . = ALIGN(8);
    } > RAM
}

Stack Management ¶

Stack Allocation ¶

FreeRTOS Task Stacks:

/* Create task with specific stack size */
osal_task_config_t config = {
    .name = "my_task",
    .func = my_task_function,
    .arg = NULL,
    .stack_size = 1024,  /* 1KB stack */
    .priority = OSAL_PRIORITY_NORMAL,
};

osal_task_handle_t task;
osal_task_create(&config, &task);

Stack Size Guidelines:

Task Type	Recommended Size
Minimal	256-512 bytes
Normal	512-1024 bytes
Complex	1024-2048 bytes
With printf	+512 bytes
With FPU	+128 bytes

Stack Usage Monitoring ¶

Check Stack High Water Mark:

void monitor_stack_usage(void)
{
    osal_task_handle_t current = osal_task_get_current();
    uint32_t high_water = osal_task_get_stack_high_water(current);
    uint32_t stack_size = osal_task_get_stack_size(current);
    uint32_t used = stack_size - high_water;
    uint32_t percent = (used * 100) / stack_size;

    LOG_INFO("Task: %s", osal_task_get_name(current));
    LOG_INFO("Stack: %lu/%lu bytes (%lu%% used)",
             used, stack_size, percent);

    if (percent > 80) {
        LOG_WARN("Stack usage high - consider increasing size");
    }
}

Stack Overflow Detection:

/* FreeRTOS stack overflow hook */
void vApplicationStackOverflowHook(TaskHandle_t xTask, char* pcTaskName)
{
    LOG_FATAL("Stack overflow in task: %s", pcTaskName);

    /* Dump task information */
    LOG_FATAL("Task handle: %p", xTask);

    /* Enter safe state */
    __disable_irq();
    while (1) {
        /* Halt system */
    }
}

Stack Canaries:

/* Enable stack protection */
#define STACK_CANARY 0xDEADBEEF

typedef struct {
    uint32_t canary_start;
    uint8_t stack[1024];
    uint32_t canary_end;
} protected_stack_t;

void check_stack_canary(protected_stack_t* stack)
{
    if (stack->canary_start != STACK_CANARY ||
        stack->canary_end != STACK_CANARY) {
        LOG_FATAL("Stack corruption detected!");
    }
}

Heap Management ¶

Dynamic Allocation ¶

OSAL Memory Functions:

/* Allocate memory */
void* ptr = osal_malloc(1024);
if (!ptr) {
    LOG_ERROR("Memory allocation failed");
    return;
}

/* Use memory */
memset(ptr, 0, 1024);

/* Free memory */
osal_free(ptr);

Aligned Allocation:

/* Allocate aligned memory (for DMA) */
void* aligned_ptr = osal_malloc_aligned(1024, 32);
if (!aligned_ptr) {
    LOG_ERROR("Aligned allocation failed");
    return;
}

/* Use for DMA */
dma_transfer(aligned_ptr, 1024);

/* Free aligned memory */
osal_free_aligned(aligned_ptr);

Heap Monitoring ¶

Check Heap Usage:

void monitor_heap_usage(void)
{
    size_t free_heap = osal_get_free_heap_size();
    size_t min_free = osal_get_minimum_ever_free_heap_size();
    size_t total_heap = configTOTAL_HEAP_SIZE;
    size_t used = total_heap - free_heap;
    uint32_t percent = (used * 100) / total_heap;

    LOG_INFO("Heap Usage:");
    LOG_INFO("  Total: %zu bytes", total_heap);
    LOG_INFO("  Used: %zu bytes (%lu%%)", used, percent);
    LOG_INFO("  Free: %zu bytes", free_heap);
    LOG_INFO("  Min Free Ever: %zu bytes", min_free);

    if (percent > 90) {
        LOG_WARN("Heap usage critical!");
    }

    if (min_free < 1024) {
        LOG_WARN("Heap has been nearly exhausted!");
    }
}

Periodic Monitoring:

void heap_monitor_task(void* arg)
{
    while (1) {
        monitor_heap_usage();
        osal_task_delay(10000);  /* Check every 10 seconds */
    }
}

Memory Pools ¶

Fixed-Size Pools ¶

Create Memory Pool:

/* Define pool for message structures */
typedef struct {
    uint32_t id;
    uint8_t data[64];
} message_t;

#define POOL_SIZE 10

osal_pool_handle_t message_pool;

void init_message_pool(void)
{
    osal_pool_create(&message_pool, POOL_SIZE, sizeof(message_t));
}

Use Memory Pool:

void send_message(uint32_t id, const uint8_t* data, size_t len)
{
    /* Allocate from pool */
    message_t* msg = osal_pool_alloc(message_pool);
    if (!msg) {
        LOG_ERROR("Pool exhausted");
        return;
    }

    /* Fill message */
    msg->id = id;
    memcpy(msg->data, data, len);

    /* Send message */
    osal_queue_send(msg_queue, &msg, 0);
}

void receive_message(void)
{
    message_t* msg;

    /* Receive message */
    if (osal_queue_receive(msg_queue, &msg, 1000) == OSAL_OK) {
        /* Process message */
        process_message(msg);

        /* Return to pool */
        osal_pool_free(message_pool, msg);
    }
}

Pool Statistics:

void print_pool_stats(void)
{
    uint32_t available = osal_pool_get_available(message_pool);
    uint32_t total = POOL_SIZE;
    uint32_t used = total - available;

    LOG_INFO("Message Pool:");
    LOG_INFO("  Total: %lu", total);
    LOG_INFO("  Used: %lu", used);
    LOG_INFO("  Available: %lu", available);
}

Static Allocation ¶

Compile-Time Allocation ¶

Static Buffers:

/* Static allocation - no runtime overhead */
#define MAX_DEVICES 10
static device_t devices[MAX_DEVICES];
static size_t device_count = 0;

device_t* allocate_device(void)
{
    if (device_count >= MAX_DEVICES) {
        return NULL;
    }
    return &devices[device_count++];
}

Static RTOS Objects:

/* FreeRTOS static allocation */
#define TASK_STACK_SIZE 1024

static StackType_t task_stack[TASK_STACK_SIZE];
static StaticTask_t task_tcb;

void create_static_task(void)
{
    TaskHandle_t task = xTaskCreateStatic(
        task_function,
        "static_task",
        TASK_STACK_SIZE,
        NULL,
        tskIDLE_PRIORITY + 1,
        task_stack,
        &task_tcb
    );
}

Memory Fragmentation ¶

Causes and Prevention ¶

Fragmentation Example:

Initial:  [--------FREE--------]

Alloc A:  [AAA-----FREE--------]
Alloc B:  [AAABBB--FREE--------]
Alloc C:  [AAABBBCC-FREE-------]

Free B:   [AAA---CC-FREE-------]  ← Fragmentation!

Alloc D:  Can't allocate 4 bytes even though 6 bytes free

Prevention Strategies:

Use Memory Pools - Fixed-size allocations
Allocate Once - Allocate at startup, never free
Use Static Allocation - No fragmentation
Allocate in Order - Allocate large blocks first
Avoid Frequent Alloc/Free - Reuse buffers

Example: Buffer Reuse:

/* Bad: Allocate/free repeatedly */
void process_data_bad(void)
{
    for (int i = 0; i < 100; i++) {
        uint8_t* buffer = malloc(1024);
        process_buffer(buffer);
        free(buffer);  /* Causes fragmentation */
    }
}

/* Good: Reuse buffer */
void process_data_good(void)
{
    uint8_t* buffer = malloc(1024);
    if (!buffer) return;

    for (int i = 0; i < 100; i++) {
        process_buffer(buffer);
    }

    free(buffer);
}

Memory Debugging ¶

Leak Detection ¶

Track Allocations:

#ifdef DEBUG_MEMORY

typedef struct {
    void* ptr;
    size_t size;
    const char* file;
    int line;
} alloc_info_t;

#define MAX_ALLOCS 100
static alloc_info_t alloc_table[MAX_ALLOCS];
static size_t alloc_count = 0;

void* debug_malloc(size_t size, const char* file, int line)
{
    void* ptr = malloc(size);
    if (ptr && alloc_count < MAX_ALLOCS) {
        alloc_table[alloc_count].ptr = ptr;
        alloc_table[alloc_count].size = size;
        alloc_table[alloc_count].file = file;
        alloc_table[alloc_count].line = line;
        alloc_count++;
    }
    return ptr;
}

void debug_free(void* ptr)
{
    for (size_t i = 0; i < alloc_count; i++) {
        if (alloc_table[i].ptr == ptr) {
            /* Remove from table */
            alloc_table[i] = alloc_table[--alloc_count];
            break;
        }
    }
    free(ptr);
}

void check_memory_leaks(void)
{
    if (alloc_count > 0) {
        LOG_WARN("Memory leaks detected: %zu allocations",
                 alloc_count);
        for (size_t i = 0; i < alloc_count; i++) {
            LOG_WARN("  %p (%zu bytes) at %s:%d",
                     alloc_table[i].ptr,
                     alloc_table[i].size,
                     alloc_table[i].file,
                     alloc_table[i].line);
        }
    }
}

#define malloc(size) debug_malloc(size, __FILE__, __LINE__)
#define free(ptr) debug_free(ptr)

#endif

Memory Corruption Detection ¶

Guard Bytes:

#define GUARD_PATTERN 0xDEADBEEF

typedef struct {
    uint32_t guard_start;
    uint8_t data[256];
    uint32_t guard_end;
} guarded_buffer_t;

void init_guarded_buffer(guarded_buffer_t* buf)
{
    buf->guard_start = GUARD_PATTERN;
    buf->guard_end = GUARD_PATTERN;
}

bool check_guarded_buffer(guarded_buffer_t* buf)
{
    if (buf->guard_start != GUARD_PATTERN) {
        LOG_ERROR("Buffer underflow detected!");
        return false;
    }
    if (buf->guard_end != GUARD_PATTERN) {
        LOG_ERROR("Buffer overflow detected!");
        return false;
    }
    return true;
}

Memory Fill Patterns:

/* Fill freed memory with pattern */
void debug_free_with_pattern(void* ptr, size_t size)
{
    memset(ptr, 0xDD, size);  /* 0xDD = "Dead" */
    free(ptr);
}

/* Detect use-after-free */
void check_for_use_after_free(void* ptr, size_t size)
{
    uint8_t* bytes = (uint8_t*)ptr;
    for (size_t i = 0; i < size; i++) {
        if (bytes[i] == 0xDD) {
            LOG_ERROR("Use-after-free detected at offset %zu", i);
            return;
        }
    }
}

Best Practices ¶

Prefer Static Allocation * Deterministic behavior * No fragmentation * Faster allocation * Suitable for safety-critical systems
Use Memory Pools * Fixed-size allocations * Fast allocation/deallocation * No fragmentation * Predictable behavior
Minimize Dynamic Allocation * Allocate at startup * Reuse buffers * Avoid frequent alloc/free * Use stack when possible
Monitor Memory Usage * Track stack high water marks * Monitor heap usage * Check for leaks * Set up alerts
Handle Allocation Failures * Always check return values * Have fallback strategies * Log failures * Graceful degradation
Align Data Properly * Use aligned allocations for DMA * Pack structures carefully * Consider cache line alignment * Use compiler attributes
Test Memory Limits * Test with low memory * Test allocation failures * Test fragmentation scenarios * Use memory debugging tools

Memory Optimization Checklist ¶

Design Phase:

[ ] Estimate memory requirements
[ ] Choose allocation strategy
[ ] Plan for worst-case usage
[ ] Design for determinism

Implementation Phase:

[ ] Use appropriate allocation method
[ ] Check all allocation failures
[ ] Free all allocated memory
[ ] Avoid memory leaks

Testing Phase:

[ ] Test with memory limits
[ ] Monitor memory usage
[ ] Check for leaks
[ ] Verify stack sizes

Optimization Phase:

[ ] Reduce dynamic allocation
[ ] Optimize data structures
[ ] Pack structures
[ ] Use memory pools

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