Storing a variable into FAR memory

[Benj]

Hello,

On some dsPIC and PIC24 devices there are two types of RAM memory due to bus limitations, near and far. Near memory is fast to access but is limited by the size of the internal bus. Far memory is much less limited but means additional overhead when fetching or storing values.

FarMem.jpg (62.3 KiB) Viewed 2900 times

When creating Flowcode variables these will default to being stored in near memory to optimise the time to fetch and store values. If you run out of room in near memory then you may get a message like this during compilation.

C:\Users\BenR\Desktop\FarMem.o: Link Error: Could not allocate section .nbss, size = 4118 bytes, attributes = bss near
Link Error: Could not allocate data memory

To tell the compiler to move the variable into FAR memory in C you would do something like this.

__attribute__((far)) unsigned char data[4096];

The Flowcode variable defaults to something like this.

MX_GLOBAL MX_UINT8 FCV_DATA[FCVsz_DATA]; // Large Data Array

The AdVar event is used during the compile to C process to take the variables from the variable manager and generate the necessary C code. By calling a macro to run on this event we can tailor the way the variables are added to the C and add in the cast to FAR memory.

AdVar.jpg (26.6 KiB) Viewed 2905 times

The first thing we need to do in the macro is compare the name of the variable that is currently being processed with the name of the variable we want to store into FAR memory. The string compare function returns 0 if the two strings match up so in the No branch of the decision we then add the "__attribute__((far)) " string to the expansion string.

EvAddVar.jpg (50.89 KiB) Viewed 2906 times

After saving the project and refreshing the C code window the variable now looks like this.

__attribute__((far)) MX_GLOBAL MX_UINT8 FCV_DATA[FCVsz_DATA]; // Large Data Array

And during compilation we now get this.

section address alignment gaps total length (dec)
------- ------- -------------- -------------------
.nbss 0x1000 0 0x16 (22)
.bss 0x1016 0 0x1000 (4096)
.heap 0x2016 0 0x100 (256)

Total data memory used (bytes): 0x1116 (4374) 15%
...
HEX file creation successful!

Here is the file as a demonstration.

FarMem.fcfx

(5.73 KiB) Downloaded 273 times

This info is also now available as an example for the AddVar event in the v6 Wiki: http://www.matrixmultimedia.com/wiki/in ... r#Examples