ROM-RAM Copying

In embedded programming, it is common that data or code of a program residing in ROM gets copied into RAM at runtime.

To indicate such data or code, use the LCF to assign it two addresses:

• The memory segment specifies the intended location in RAM
• The sections segment specifies the resident location in ROM, via its AT (address) parameter

For example, suppose you want to copy all initialized data into RAM at runtime. At runtime, the system loads the .main_data section containing the initialized data to RAM address 0x80000, but until runtime, this section remains in ROM. The following listing shows part of the corresponding LCF.

# ROM location: address 0x0
# RAM location: address 0x800000

# For clarity, no alignment directives in this listing

MEMORY {

  TEXT (RX) : ORIGIN = 0x0, LENGTH = 0

  DATA (RW) : ORIGIN = 0x800000, LENGTH = 0

}

SECTIONS{

  .main :

  {

    *(.text)

    *(.rodata)

  } > TEXT

# Locate initialized data in ROM area at end of .main.

  .main_data : AT( ADDR(.main) + SIZEOF(.main) )

  {

    *(.data)

    *(.sdata)

    *(.sbss)

  } > DATA

  .uninitialized_data:

  {

    *(SCOMMON)

    *(.bss)

    *(COMMON)

  } >> DATA

For program execution to copy the section from ROM to RAM, a copy table as shown in the following listing, must supply the information that the program needs at runtime. This copy table, which the symbol __S_romp identifies, contains a sequence of three word values per entry:

• ROM start address
• RAM start address
• size

The last entry in this table must be all zeros: this is the reason for the three lines WRITEW(0) ; before the table closing brace character.

# Locate ROM copy table into ROM after initialized data
_romp_at = _main_ROM + SIZEOF(.main_data);

 .romp : AT (_romp_at)

 {

   __S_romp = _romp_at;

   WRITEW(_main_ROM);          #ROM start address

   WRITEW(ADDR(.main_data));   #RAM start address

   WRITEW(SIZEOF(.main_data)); #size

   WRITEW(0);

   WRITEW(0);

   WRITEW(0);

 }

 __SP_INIT = . + 0x4000;  # set stack to 16kb

 __heap_addr = __SP_INIT; # heap grows opposite stack direction

 __heap_size = 0x10000;   # set heap to 64kb

}                         # end SECTIONS segment

                          # end LCF