DevHeads IoT Integration Server•10mo ago

Trying to access the sensor's registers using memory-mapped I/O in a flat memory model

Hi, I'm working on a small embedded system project using x86-64 assembly on an Intel Atom E3845 microprocessor. The program aims to read temperature data from an Adafruit BMP280 sensor connected with I2C

Solution:

No more error message, it worked:
```global _start section .text _start:...

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9 Replies

Marvee Amasi•10mo ago

I'm trying to access the sensor's registers using memory-mapped I/O in a flat memory model, and I'm encountering a segmentation fault when I run the program, saying:

Segmentation fault (core dumped)

Segmentation fault (core dumped)

My relevant code :

global _start

section .text
    _start:
        ; I2C communication registers set up...
        
        ; base address for BMP280 registers setting
        movw $0x10000, %ax  ;  base address setting

        ; reading temp data register 
        movb (%rax), %bl  ; reading byte from temp register

        ; processing my temp data...

        movb $1, %eax
        int $0x80         ; exit system call

section .data
        ; data segment...
        message db "Temp data read", 0x0

global _start

section .text
    _start:
        ; I2C communication registers set up...
        
        ; base address for BMP280 registers setting
        movw $0x10000, %ax  ;  base address setting

        ; reading temp data register 
        movb (%rax), %bl  ; reading byte from temp register

        ; processing my temp data...

        movb $1, %eax
        int $0x80         ; exit system call

section .data
        ; data segment...
        message db "Temp data read", 0x0

Marvee Amasi•10mo ago

Can you help me identify the cause of the segmentation fault and suggest appropriate methods to access the BMP280 sensor registers in x86-64 assembly, considering the limitations of a flat memory model? Additionally, are there recommended practices or libraries for I2C sensor interaction on Intel Atom processors in a Linux environment? @Helper

Enthernet Code•10mo ago

Hi @Marvee Amasi , it looks like you’re encountering a segmentation fault due to trying to access memory that your program doesn’t have permission to access.

Marvee Amasi•10mo ago

What am I doing wrong?

Enthernet Code•10mo ago

Incorrect Use of Memory-Mapped I/O in User Space and Incorrect Register Access

Marvee Amasi•10mo ago

Yup @Enthernet Code I had to implement memory-mapped I/O using the mmap system call

Enthernet Code•10mo ago

@Marvee Amasi Accessing hardware registers directly through memory-mapped I/O in user-space is not allowed in modern operating systems like Linux. This is due to the protection mechanisms in place to prevent unauthorized access to hardware resources.The segmentation fault occurs because the memory address 0x10000 you are trying to access is not allocated to your program, and hence, the operating system prevents access.

Solution

Marvee Amasi•10mo ago

No more error message, it worked:

global _start

section .text
_start:
    ; I2C communication registers set up...

    ; 
    mov $0, %eax  ; opening system call
    mov $0xFD, %ebx ; open file descriptor for /dev/mem
    mov $0, %ecx  ; 
    mov $_dev_mem, %edx  ; 
    int $0x80     ; system call

    ; checking for successful open
    cmp $0, %eax
    jl .open_failed  ; if fails

    ; Mapping my memory region 
    mov $9, %eax   ; mmap system call
    mov %eax, %ebx  ; file descriptor from open
    mov $0x1000, %ecx ; 
    mov $0x1000, %edx ; 
    mov $0x1, %esi  ; prot - PROT_READ
    mov $0x0, %edi  ; flags
    mov $0, %ebp   ; offset 
    int $0x80     ; system call

    ; if mapping is successful
    cmp $0, %eax
    jl .mmap_failed  ; if mapping fails 

    ; accessing registers using the mapped address
    movb (%rax), %bl  ; reading byte from temp register

    ; processing my temp data...

    ; Unmapping memory region before exiting 
    mov $17, %eax  ; munmap system call
    mov %eax, %ebx  ; mapped address
    mov $0x1000, %ecx ; 
    int $0x80     ; system call

    mov $1, %eax
    int $0x80         ; exit system call

.open_failed:
    ; Handle error message 
    ...

.mmap_failed:
    ; Handle mapping error 
    ...

section .data
    _dev_mem: db "/dev/mem", 0x0  ; 
    message db "Temp data read", 0x0

global _start

section .text
_start:
    ; I2C communication registers set up...

    ; 
    mov $0, %eax  ; opening system call
    mov $0xFD, %ebx ; open file descriptor for /dev/mem
    mov $0, %ecx  ; 
    mov $_dev_mem, %edx  ; 
    int $0x80     ; system call

    ; checking for successful open
    cmp $0, %eax
    jl .open_failed  ; if fails

    ; Mapping my memory region 
    mov $9, %eax   ; mmap system call
    mov %eax, %ebx  ; file descriptor from open
    mov $0x1000, %ecx ; 
    mov $0x1000, %edx ; 
    mov $0x1, %esi  ; prot - PROT_READ
    mov $0x0, %edi  ; flags
    mov $0, %ebp   ; offset 
    int $0x80     ; system call

    ; if mapping is successful
    cmp $0, %eax
    jl .mmap_failed  ; if mapping fails 

    ; accessing registers using the mapped address
    movb (%rax), %bl  ; reading byte from temp register

    ; processing my temp data...

    ; Unmapping memory region before exiting 
    mov $17, %eax  ; munmap system call
    mov %eax, %ebx  ; mapped address
    mov $0x1000, %ecx ; 
    int $0x80     ; system call

    mov $1, %eax
    int $0x80         ; exit system call

.open_failed:
    ; Handle error message 
    ...

.mmap_failed:
    ; Handle mapping error 
    ...

section .data
    _dev_mem: db "/dev/mem", 0x0  ; 
    message db "Temp data read", 0x0

Had to dig deep with some resources using mmap

Marvee Amasi•10mo ago

Thanks man @Enthernet Code

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Trying to access the sensor's registers using memory-mapped I/O in a flat memory model

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