What is using up all the RAM in my program?

Question

I am running a program on the ATtiny and for some reason I am way over the 512 bytes of RAM that are available to me. This is confusing because when I did the math by hand I should have 432 bytes. Here is my code:

#include <SoftwareSerial.h>

#define FASTADC 1

// defines for setting and clearing register bits
#ifndef cbi
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#endif
#ifndef sbi
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
#endif

SoftwareSerial mySerial(0,1);    //This uses 47 bytes of RAM

byte i = 0;      This uses 1 byte of RAM

byte pixelsArray1[128];   //Arrays use 128 bytes of RAM each
byte pixelsArray2[128];
byte pixelsArray3[128];

void setup() 
{
#if FASTADC
// set prescale to 16
sbi(ADCSRA,ADPS2);
cbi(ADCSRA,ADPS1);
cbi(ADCSRA,ADPS0);
#endif

pinMode(1,OUTPUT);        //Setting the 1 pin to be used for output 
pinMode(0,OUTPUT);        //Setting the 0 pin to be used for output
pinMode(A3,INPUT);         //Input for Camera 3
pinMode(A2,INPUT);         //Input for Camera 2
pinMode(A1,INPUT);         //Input for Camera 1

mySerial.begin(9600);     //Setting the data transfer rate
}

It's telling me that I am using 510 Bytes of memory but my math gave me 432 bytes of RAM. Does anyone see what I'm missing?

Answer 1

There is a lot you are missing. Such as:

1. SoftwareSerial is using RAM.
2. The system stack is using RAM.
3. The Arduino core software is using RAM.

There's plenty more going on than just your sketch. For instance, where do you think it stores the current millis() and micros() counts? Where do you think incoming serial data gets buffered? Where do you think your registers, PC, etc, get stashed when a function (such as setup()) is called? All that takes RAM.