In many of user applications we use a CRC/checksum to verify that the code/flash on the target is not modified. For this, not only the code/data in flash counts, but as well all the unused gaps in the memory map. Instead to leave it up to the flasher/debugger (which usually erases it to 0xFF), I want to fill it with my pattern. For example 0xAA.
How to implement?
There may be several ways, but I think the easiest way is to modify linker file. the linker file structures are different for different MCUs. For example, CW for 8bit/16bit and Coldfire V1 uses PRM file; while CW for kinetis Gnu compiler uses ld file. We will talk about each of the case separately.
1. CW for 8bit/16bit and Coldfire V1.
1.1 . NO FILL command involved.
We define a segment in prm file
MYCONST_ROM = READ_ONLY 0x0870 TO 0x08FF;
Then allocate MYCONST into it inside PLACEMENT.
MYCONST INTO MYCONST_ROM;
In C code, we only define my_const_var(0xCCDD) at this range.
#pragma CONST_SEG MYCONST
const unsigned int my_const_var = 0xCCDD;
#pragma CONST_SEG DEFAULT
After build, we will see only 0xCCDD in this area in generated s19 file:
S1050870CCDDD9
1.2 use FILL command to fill unused area of MYCONST_ROM
With FILL command we can fill unused area of a section. Add “FILL 0xAA” :
MYCONST_ROM = READ_ONLY 0x0870 TO 0x08FF FILL 0XAA;
Rebuild the project. we will see the rest of the field of MYCONST_ROM is filled in S19 file:
S1230870CCDDAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACF
S1230890AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA04
S12308B0AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAE4
S12308D0AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAC4
S11308F0AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA54
2. CW for kinetis GNU compiler:
2.1 . No fill() command involed.
Define segment in MEMORY section:
my_section (rx) : ORIGIN = 0X00000800, LENGTH = 0x20
define output SECTION:
.myData :
{
. = ALIGN(4);
KEEP(*(.myData))
. = ALIGN(4);
}> my_section
In C code, define const variable “my_const” (0xCCCCDDDD)
const int my_const __attribute__((section(".myData"))) = 0xCCCCDDDD;
After build, we will see only 0xCCCCDDDD in this area in generated s19 file:
S1070800DDDDCCCC9E
2.2 Use FILL(0xaa) command to fill unused area of my_section with 0xAA.
Here is the modified code in ld file, I highlight the code I add:
.myData :
{
. = ALIGN(4);
KEEP(*(.myData))
. = ALIGN(4);
FILL(0xaa)
. = ORIGIN(my_section) + LENGTH(my_section) ;
}> my_section
Rebuild the project. we will see the rest of the field of my_section is filled with 0xAA in S19 file:
S1130800DDDDCCCCAAAAAAAAAAAAAAAAAAAAAAAA9A
S1130810AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA34
