Sections (.bss) , (.rodata) and (.data) ... where and how to place them?

I've been struggling with (.data), (.bss) and (.rodata) sections. At the end of this message, I have some issues I couldn't figure out. Any help will be greatly appreciated.

I did some testing and here are the results. Hope this information is helpful to anybody else.

Some ELF sections definitions (from targeting manual):

========================================================

 (.bss) => segment for uninitialized data. The stationery init code zeroes this area at startup.

 (.data) => segment for initialized data here by default.

 (.rodata) => segment for constant data, if the project has the write constant data to .rodata section checkbox enabled in target settings.

By default, zero-initialized data is put into the .bss section by the compiler (zeroed by INIT code). This behavior can be overriden by using the #pragma explicit_zero_data

Results compiling a C file, using Codewarrior 5.1:

========================================================

(comments indicate where the variable/constant was placed, taken from ELF symbol table)

// **** initialized stuff ****

const unsigned int VAR1[3]  = { 0x110A, 0x110B, 0x110C}; // .rodata

const          int VAR2[3]  = { 0x220A, 0x220B, 0x220C}; // .rodata

      unsigned int VAR3[3]  = { 0x330A, 0x330B, 0x330C}; // .data

               int VAR4[3]  = { 0x440A, 0x440B, 0x440C}; // .data

const unsigned int VAR5     = 0x5555;   // .rodata

const          int VAR6     = 0x6666;   // .rodata

      unsigned int VAR7     = 0x7777;   // .data

               int VAR8     = 0x8888;   // .data

// **** zero-initialized stuff ****

const unsigned int VAR9[3]  = { 0x0000, 0x0000, 0x0000}; // .rodata

const          int VAR10[3] = { 0x0000, 0x0000, 0x0000}; // .rodata

      unsigned int VAR11[3] = { 0x0000, 0x0000, 0x0000}; // .bss

               int VAR12[3] = { 0x0000, 0x0000, 0x0000}; // .bss

const unsigned int VAR13    = 0x0000;   // .rodata

const          int VAR14    = 0x0000;   // .rodata

      unsigned int VAR15    = 0x0000;   // .bss

               int VAR16    = 0x0000;   // .bss

// **** uninitialized stuff ****

      unsigned int VAR17[3] ;   // .bss

               int VAR18[3] ;   // .bss

      unsigned int VAR19    ;   // .bss

               int VAR20    ;   // .bss 

Observations:

========================================================

Constants: as expected, all of them are placed into (.rodata), even zero-initialized. These are to be loaded with application.

Variables: as expected, initialized stuff goes to (.data) and will be loaded to DSP with application. Zero-initialized stuff goes to (.bss) and will not be loaded with application. Instead, they'll be zeroed by DSP initialize code. Uninitialized variables also go to (.bss).

Stuff that I couldn't figure out:

========================================================

Where and how to place these sections in memory?

For (.rodata) sections, I would place them in (.x_flash_ROM). They are supposed to be constants, they should be downloaded with application and remain there when power goes off. Right?

For (.bss) sections, I would place them in (.x_RAM). They are supposed to be R/W and will be initialized after /RESET. Therefore, uninitialized data and zero-initialized data should go here. Right?

For (.data)... I read a thousand times "ROM to RAM Copying" in targeting. Still can't figure out. Here's my thinking: (.data) holds initialized variables. I would want them to be downloaded to ROM (to save them when power goes off) and copied to RAM when using them (to be able to R/W).

Questions:

a) Why "ROM to RAM Copying" specifies to move (.data) to .p_ROM and not .x_ROM ? Why not copying from .xROM to .xRAM ?

b) "Targeting" specifies this LCF:

----------------------------

MEMORY {

  .text (RWX) : ORIGIN = 0x8000, LENGTH = 0x0    # code (P)

  .data (RW)  : ORIGIN = 0x3000, LENGTH = 0x0    # data (X)-> RAM

}

SECTIONS{

F__ROM_Address = 0x1000;        # ROM Starting Address

  .main_application :

  {

    # .text sections

    *(.text)

  } > .text

  .data : AT( F__ROM_Address )  # Start data at 0x1000 -> ROM

  {

    # .data sections

    F_Begin_Data = .;           # Get start location for RAM

    *(.data)                    # Write data to the section (ROM)

    F_End_Data = .;             # Get end location for RAM

    # .bss sections

    * (.bss)

  } > .data

}

----------------------------

b.1) "F__ROM_Address = 0x1000" does not specify if it is .xROM, .xRAM, etc. What does "AT( F__ROM_Address )" mean ? Shouldn't it be like "F_ROM_Address = .;" inside a (.text) section?

b.2) I assume that AT means "put the following sections in F__ROM_Address, but declare them as .data in .xRAM. Is this so?

b.3) In my C file, I should do a :

   memcpy( (unsigned long *)&__Begin_Data,

           (const unsigned long *)&__ROMAddress,

           dataLen );

But how to tell the compiler that one address corresponds to .pROM and the other to .xRAM? is it embedded in __Begin_Data and __ROMAddress ?

I apologize for the extended size of this post. Any help on this will be greatly appreciated. It's 1:30am and I should be sleeping by now, but CodeWarrior has been keeping me awake till late with this mess... I hope I can solve this soon and get my life back to normal :)

Regards,

Mariano Filippa

From: Dobsik Lubomir-RC085C

To: Mariano Filippa

Sent: Tuesday, January 06, 2004 9:33 AM

Subject: RE: [motoroladsp] Sections (.bss) , (.rodata) and (.data) ... where and how to place them?

well generally, you can choose whether you want your data to be stored in xflash or pflash, it's up to the initialization, startup, routine to do the proper data move...

 

in the code, the variables will be referenced by the address of the data section they are placed in (plus offset), but physically, they can be stored at any address (this is what the 'AT' command is good for).. so, for example, you want to have the initialized data stored in the xflash

 

.app_data : AT( F__ROM_Address )  # Start data at 0x1000 -> ROM

{

    # .data sections

    F_Begin_Data = .;           # Get start location for RAM

    *(.data)                    # Write data to the section (ROM)

    F_End_Data = .;             # Get end location for RAM

} > .data

 

.app_bss :

{

    # .bss sections

    * (.bss)

 

} >> .data             #append to the .data memory segment

 

 

 

in the startup code, you must move the (F_End_Data - F_Begin_Data) words from the address "F__ROM_Address" to "F_Begin_Data" so the variables get initialized.

 

the second case, when the init values are stored in the pflash is rather more involved. you must declare another memory segment in the p-memory, spanning the same addresses as the data memory segment there the variables should reside (an image memory segment), you will take advantage of the fact that codewarrior doesn't make difference between x-mem and p-mem pointers.

 

.app_data : AT (__data_ROM_addr) # address of the data in p-flash
{

  F_Begin_Data = .;
  * (.data)
  F_End_Data = .;
} > .p_internal_RAM_image        # spans the same addresses as the .data memory segment

.app_bss :
{
 . = . + F_End_Data - F_Begin_Data;   # skip the init-ed variables

  # .bss sections

  * (.bss)

....

} > .data

 

 

if something was still unclear, don't heasitate to ask me,

 

Lubomir

-----Original Message-----
From: Mariano Filippa [mailto:m...@fibertel.com.ar]
Sent: 6. ledna 2004 7:44
To: m...@yahoogroups.com
Subject: [motoroladsp] Sections (.bss) , (.rodata) and (.data) ... where and how to place them?

I've been struggling with (.data), (.bss) and (.rodata) sections. At the end of this message, I have some issues I couldn't figure out. Any help will be greatly appreciated.

 

I did some testing and here are the results. Hope this information is helpful to anybody else.

 

 

Some ELF sections definitions (from targeting manual):

========================================================

 

 (.bss) => segment for uninitialized data. The stationery init code zeroes this area at startup.

 

 (.data) => segment for initialized data here by default.

 

 (.rodata) => segment for constant data, if the project has the write constant data to .rodata section checkbox enabled in target settings.

 

By default, zero-initialized data is put into the .bss section by the compiler (zeroed by INIT code). This behavior can be overriden by using the #pragma explicit_zero_data

 

Results compiling a C file, using Codewarrior 5.1:

========================================================

 

(comments indicate where the variable/constant was placed, taken from ELF symbol table)

 

// **** initialized stuff ****

 

const unsigned int VAR1[3]  = { 0x110A, 0x110B, 0x110C}; // .rodata

const          int VAR2[3]  = { 0x220A, 0x220B, 0x220C}; // .rodata

      unsigned int VAR3[3]  = { 0x330A, 0x330B, 0x330C}; // .data

               int VAR4[3]  = { 0x440A, 0x440B, 0x440C}; // .data

 

const unsigned int VAR5     = 0x5555;   // .rodata

const          int VAR6     = 0x6666;   // .rodata

      unsigned int VAR7     = 0x7777;   // .data

               int VAR8     = 0x8888;   // .data

 

// **** zero-initialized stuff ****

              

const unsigned int VAR9[3]  = { 0x0000, 0x0000, 0x0000}; // .rodata

const          int VAR10[3] = { 0x0000, 0x0000, 0x0000}; // .rodata

      unsigned int VAR11[3] = { 0x0000, 0x0000, 0x0000}; // .bss

               int VAR12[3] = { 0x0000, 0x0000, 0x0000}; // .bss

 

const unsigned int VAR13    = 0x0000;   // .rodata

const          int VAR14    = 0x0000;   // .rodata

      unsigned int VAR15    = 0x0000;   // .bss

               int VAR16    = 0x0000;   // .bss

 

// **** uninitialized stuff ****

              

      unsigned int VAR17[3] ;   // .bss

               int VAR18[3] ;   // .bss

 

      unsigned int VAR19    ;   // .bss

               int VAR20    ;   // .bss 

 

Observations:

========================================================

 

Constants: as expected, all of them are placed into (.rodata), even zero-initialized. These are to be loaded with application.

 

Variables: as expected, initialized stuff goes to (.data) and will be loaded to DSP with application. Zero-initialized stuff goes to (.bss) and will not be loaded with application. Instead, they'll be zeroed by DSP initialize code. Uninitialized variables also go to (.bss).

 

Stuff that I couldn't figure out:

========================================================

 

Where and how to place these sections in memory?

 

For (.rodata) sections, I would place them in (.x_flash_ROM). They are supposed to be constants, they should be downloaded with application and remain there when power goes off. Right?

 

For (.bss) sections, I would place them in (.x_RAM). They are supposed to be R/W and will be initialized after /RESET. Therefore, uninitialized data and zero-initialized data should go here. Right?

 

For (.data)... I read a thousand times "ROM to RAM Copying" in targeting. Still can't figure out. Here's my thinking: (.data) holds initialized variables. I would want them to be downloaded to ROM (to save them when power goes off) and copied to RAM when using them (to be able to R/W).

 

Questions:

 

a) Why "ROM to RAM Copying" specifies to move (.data) to .p_ROM and not .x_ROM ? Why not copying from .xROM to .xRAM ?

 

b) "Targeting" specifies this LCF:

----------------------------

MEMORY {

  .text (RWX) : ORIGIN = 0x8000, LENGTH = 0x0    # code (P)

  .data (RW)  : ORIGIN = 0x3000, LENGTH = 0x0    # data (X)-> RAM

}

 

SECTIONS{

 

F__ROM_Address = 0x1000;        # ROM Starting Address

 

  .main_application :

  {

    # .text sections

    *(.text)

  } > .text

 

  .data : AT( F__ROM_Address )  # Start data at 0x1000 -> ROM

  {

    # .data sections

    F_Begin_Data = .;           # Get start location for RAM

    *(.data)                    # Write data to the section (ROM)

    F_End_Data = .;             # Get end location for RAM

 

    # .bss sections

    * (.bss)

 

  } > .data

}

----------------------------

 

b.1) "F__ROM_Address = 0x1000" does not specify if it is .xROM, .xRAM, etc. What does "AT( F__ROM_Address )" mean ? Shouldn't it be like "F_ROM_Address = .;" inside a (.text) section?

 

b.2) I assume that AT means "put the following sections in F__ROM_Address, but declare them as .data in .xRAM. Is this so?

 

b.3) In my C file, I should do a :

   memcpy( (unsigned long *)&__Begin_Data,

           (const unsigned long *)&__ROMAddress,

           dataLen );

But how to tell the compiler that one address corresponds to .pROM and the other to .xRAM? is it embedded in __Begin_Data and __ROMAddress ?

 

 

I apologize for the extended size of this post. Any help on this will be greatly appreciated. It's 1:30am and I should be sleeping by now, but CodeWarrior has been keeping me awake till late with this mess... I hope I can solve this soon and get my life back to normal :)

 

Regards,

Mariano Filippa

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