CODEWITHSUNDEEP
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Phebus40
Phebus40

Reputation: 173

Variables .BSS section

Uninitialized variables are included in the BSS section.

First question : Will they stay there when they are assigned?

Second question/problem : I want to load this section in an external SDRAM. Variables are properly located in the memory map, but filled with strange values, and I can't store any other number into. Do you have an idea about this problem?

/* Memory Map                                                                 */
MEMORY{
    VECTORS (X)  : origin=0x00000000 length=0x00000020
    FLASH0  (RX) : origin=0x00000020 length=0x0017FFE0
    FLASH1  (RX) : origin=0x00180000 length=0x00180000
    STACKS  (RW) : origin=0x08000000 length=0x00001500
    RAM     (RW) : origin=0x08001500 length=0x0003EB00
    SDRAM   (RW) : origin=0x80000000 length=0x00800000
}

/*----------------------------------------------------------------------------*/
/* Section Configuration                                                      */
SECTIONS{
    .intvecs : {} > VECTORS
    .text    : {} > FLASH0 | FLASH1
    .const   : {} > FLASH0 | FLASH1
    .cinit   : {} > FLASH0 | FLASH1
    .pinit   : {} > FLASH0 | FLASH1
    .bss     : {} > SDRAM
    .data    : {} > RAM
    .sysmem  : {} > RAM
}

Upvotes: 1

Views: 2448

Answers (4)

Clifford
Clifford

Reputation: 93556

Re Q2, you do have to make sure that your SDRAM memory controller is correctly initialised with the correct timings and mapping for your memory device before the .bss segment is zero initialised by the runtime start-up code. You also have to ensure that your runtime start-up code is explicitly initialising the .bss segment when located in SDRAM.

Upvotes: 2

user2194434
user2194434

Reputation: 81

Q2: Details of how this must be done depend on the system you're using (are you using Linux?)

To get the .bss section where you want it, you would have to modify the linker script. e.g. for the Linux kernel that would be vmlinux.lds.

Also in Linux, the .bss section is cleared to all-zero by the bootloader, e.g. Uboot, based on the information in the elf-file. Your .bss seems to be not cleared to zero. A good place to start debugging is the bootloader. Your SDRAM may not be properly mapped in memory at the time the bootloader is clearing it (can you do succesful write-read tests while you system is running?). Or the memory protection of your CPU may prevent access.

Upvotes: 0

Ganesh
Ganesh

Reputation: 5990

Q1: Yes, variables will be in .bss section.

Q2: Can you try to move .sysmem into SDRAM and check if you can read and write the values? I suspect that you don't have the requisite accesses.

Upvotes: 0

Roddy
Roddy

Reputation: 68074

Q1 : Yes, that's where they are. Why would they move (and why would you want them to?)

http://en.wikipedia.org/wiki/.bss

Q2: I don't understand the question fully, but are you sure your SDRAM is actualy working? Does your platform have a monitor to allow you to read and write memory?

Upvotes: 1

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