# Ram vs Flash - Using large arrays

**URL:** <https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504>\
**Category:** Troubleshooting\
**Created:** [December 1, 2014, 1:16am UTC](https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504 "2014-12-01T01:16:54Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![harrisonhjones](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/harrisonhjones/32/4132_2.png) [@harrisonhjones](https://community.particle.io/u/harrisonhjones)\
**Post date:** [December 1, 2014, 1:16am UTC](https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504/1 "2014-12-01T01:16:54Z")

</div>

I have a question about RAM vs ROM.

I have a pretty simple program running on my 'Core. A few lines of code really. The problem however is that these few lines of code require a fairly huge byte array.

```
static unsigned char image[] PROGMEM = 
{
  0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
  ...
};

```

image is 5929 bytes large. When I compile I get the following error:  
`region `RAM' overflowed by 5096 bytes`  
which obviously indicates that my image is to blame. My question is, how do I fix this? I ask because the exact same code runs on a Arduino MEGA without problems. The mega has the same amount of Flash and significantly less SRAM (8KB vs 20KB). So I know this can be done, I’m just not sure how. Would it be possible to save the image file to flash somehow?

Thanks for the help.

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<div class="post-metadata">

**Author:** ![bko](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bko/32/20704_2.png) [@bko](https://community.particle.io/u/bko)\
**Post date:** [December 1, 2014, 3:02am UTC](https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504/2 "2014-12-01T03:02:11Z")

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So PROGMEM does nothing in the core compiler, I think. Try declaring your array as a global constant:

```cpp
const unsigned char image[] = 
          { 0xFF, ...
          };

```

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<div class="post-metadata">

**Author:** ![peekay123](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/peekay123/32/810_2.png) [@peekay123](https://community.particle.io/u/peekay123)\
**Post date:** [December 1, 2014, 3:56am UTC](https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504/3 "2014-12-01T03:56:16Z")

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@harrisonhjones, @bko, PROGMEM is defined as nothing since there is no equivalency in in the Core. The use of “const” place the array in flash instead of RAM. 😄

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<div class="post-metadata">

**Author:** ![harrisonhjones](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/harrisonhjones/32/4132_2.png) [@harrisonhjones](https://community.particle.io/u/harrisonhjones)\
**Post date:** [December 1, 2014, 5:03am UTC](https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504/4 "2014-12-01T05:03:30Z")

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Thanks @bko and @peekay123.

I was able to compile my code and place it on the core but the result was not the same as what I got on the mega. Instead of displaying a picture of a person I get what looks like random vertical lines

I’ve narrowed it down to what I imagine is the problem. The following command is used to read the image data:

```
pixels = pgm_read_byte_near(data + b -1) & 0xaa;

```

I imagine the use of `pgm_read_byte_near` is my problem. Suggestions?

[Edit] And NEVERMIND. The library’s functions are flexible enough to not use pgm\_read\_byte\_\* but the library itself didn’t expose this feature. Wrapped it in a #ifdef (SPARK) and it works now. Thanks both of you!

**[Edit 2] I would like to be able to update the data in memory in runtime. Is that possible? For example, I’d like to download a file over TCP and replace the image in flash. Pointers?**

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<div class="post-metadata">

**Author:** ![ScruffR](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/scruffr/32/6952_2.png) [@ScruffR](https://community.particle.io/u/ScruffR)\
**Post date:** [December 1, 2014, 7:54am UTC](https://community.particle.io/t/ram-vs-flash-using-large-arrays/8504/5 "2014-12-01T07:54:04Z")

</div>

@harrisonhjones, you might already be aware, but since this seems to be a common issue when porting Arduino libs some macros have been widely used to work around that

```auto
// just two to start with
#define pgm_read_byte(addr) (*(const unsigned char *)(addr))
#define pgm_read_word(addr) (*(const unsigned short *)(addr))

// These can be defined for the xxx_near versions, too

// There are also some prog_xxx var types that ca be treated similarly

```

Unfortunately there seems to be no such ease dropin and forget macro for `PROGMEM`, there you have to have two versions for Arduino and Spark, I think.  
But if the var is already declared as `const` it’s - as @peekay123 already said - placed in flash on the Core, so `PROGMEM` can just be `#define PROGMEM` and on Arduino `PROGMEM` will do its job for const vars just as well.