Read from 64 bit array in PROGMEM?

[steve-derr]

I had a big array of data that is stored in PROGMEM that is currently a u32. My project needs have expanded and I need to convert the array to u64. I have converted it already, but I cannot figure out how to read 64 bit values from it. In the previous version, I was using pgm_read_dword(). I cannot find a similar function to read 64 bit array entries. I thought I could read two 32 bit entries reading pgm_read_dword(index) and pgm_read_dword(index + 4 bytes), but I always just get the first 32 bits from the next array entry, rather than the last 32 bits of the 64 bit entry at index. Is there a way to get u64 data from the PROGMEM?

static const PROGMEM uint64_t LUT[250] = {.....

uint32_t myData = pgm_read_dword(LUT + index);

[david_2018]

You could use memcpy_P to copy an 8 byte block of PROGMEM into your 64 bit variable location.

[steve-derr]

@david_2018

memcpy_P

Appears to work just how I wanted. Thanks.

[westfw]

@steve-derr

I thought I could read two 32 bit entries reading pgm_read_dword(index) and pgm_read_dword(index + 4 bytes), but I always just get the first 32 bits from the next array entry

memcpy_P is fine, but I thought I'd mention that this is probably a classic "integer promotion" issue. If you have code that looks like:

   uint64_t result = (pgm_read_dword(index+4)<<32) + pgm_read_dword(index);

then the two values on the right are both uint32, and the shift and add will be done with 32bit arithmetic. (Making x<<32 zero, since all the bits are shifted out.)

You should be able to do something like:

   uint64_t result = ( ((uint64_t)pgm_read_dword(index+4)) <<32) + pgm_read_dword(index);