Memory-to-peripheral transfer in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this DMA transfer status code?

Consider this embedded C code snippet that checks the status of a DMA transfer. What will be printed?

Embedded C

#include <stdio.h>
#include <stdint.h>

#define DMA_TRANSFER_COMPLETE 1

uint8_t DMA_Status = DMA_TRANSFER_COMPLETE;

int main() {
    if (DMA_Status & DMA_TRANSFER_COMPLETE) {
        printf("Transfer Complete\n");
    } else {
        printf("Transfer Incomplete\n");
    }
    return 0;
}

A Compilation error

B Transfer Incomplete

C Transfer Complete

D No output

💡 Hint

Look at how the DMA_Status variable is checked using bitwise AND.

The DMA_Status variable has the DMA_TRANSFER_COMPLETE bit set. The if condition checks this bit using bitwise AND, so it prints 'Transfer Complete'.

❓ Predict Output

intermediate

What is the value of buffer after this memory-to-peripheral transfer simulation?

This code simulates copying data from memory to a peripheral buffer. What is the final content of buffer?

Embedded C

#include <stdio.h>
#include <string.h>

int main() {
    char memory[] = "DATA";
    char buffer[5] = {0};
    memcpy(buffer, memory, 4);
    buffer[4] = '\0';
    printf("%s", buffer);
    return 0;
}

A DATA

B Compilation error

C DAT

D DATA\0

💡 Hint

Check how memcpy copies bytes and how the null terminator is added.

memcpy copies 4 bytes from memory to buffer, which are 'D','A','T','A'. Then buffer[4] is set to '\0' to terminate the string. So printing buffer outputs 'DATA'.

🔧 Debug

advanced

Why does this DMA transfer code cause a runtime error?

Examine this code snippet for a DMA transfer setup. Why does it cause a runtime error?

Embedded C

void start_dma_transfer(uint32_t *src, uint32_t *dst, size_t size) {
    for (size_t i = 0; i < size; i++) {
        dst[i] = src[i];
    }
}

int main() {
    uint32_t source[3] = {1, 2, 3};
    uint32_t destination[3] = {0};
    start_dma_transfer(source, destination, 3);
    return 0;
}

A Compilation error due to missing header

B Null pointer dereference

C No error, runs fine

D Array index out of bounds causing undefined behavior

💡 Hint

Check the loop condition and array sizes carefully.

The loop runs from i=0 to i<=size, which means it accesses dst[3] and src[3], but arrays have indices 0 to 2. This causes out-of-bounds access and undefined behavior.

📝 Syntax

advanced

Which option fixes the syntax error in this DMA transfer initialization?

This code snippet has a syntax error. Which option fixes it?

Embedded C

typedef struct {
    uint32_t *source;
    uint32_t *destination;
    size_t length;
} DMA_Transfer;

void init_dma(DMA_Transfer *transfer) {
    transfer->source = (uint32_t *)0x20000000;
    transfer->destination = (uint32_t *)0x40000000;
    transfer->length = 256;
}

A Change '->' to '.' operator

B Add a semicolon after the first assignment line

C Remove the cast to (uint32_t *)

D Add braces around the struct initialization

💡 Hint

Look carefully at the end of each statement line.

The line 'transfer->source = (uint32_t *)0x20000000' is missing a semicolon at the end, causing a syntax error. Adding the semicolon fixes it.

🚀 Application

expert

How many bytes are transferred in this memory-to-peripheral DMA setup?

Given this DMA transfer configuration, how many bytes will be transferred?

Embedded C

typedef struct {
    uint8_t *src;
    volatile uint8_t *peripheral;
    size_t count;
} DMA_Config;

int main() {
    DMA_Config config = {
        .src = (uint8_t *)0x20001000,
        .peripheral = (volatile uint8_t *)0x40002000,
        .count = 128
    };
    // DMA transfer code here
    return 0;
}

A 128 bytes

B Cannot determine from code

C 64 bytes

D 256 bytes

💡 Hint

Look at the count field and data type size.

The count is 128 and the data type is uint8_t which is 1 byte, so total bytes transferred is 128 * 1 = 128 bytes.