Using a register value as a memory address in gcc

I am making a simple function in assembly which copies an array of bytes from one memory address to another.

The program takes 2 memory addresses (_source and _destiny), and loops over each element copying _source[i] element to _destiny[i] element. However, I can't find any information about how to use a register's value as a memory address. In this code, $[eax] and $[ecx] are supposed to be used as memory addresses.

void Copy(void const* _source, void* _destiny, size_t _byteSize)  {      char* destiny = new char[_byteSize];      for (size_t i = 0; i < _byteSize; i++)      {          __asm          (""              "mov eax, %[_source]\n\t"              "add eax, %[i]\n\t"              "mov bl, $[eax]\n\t"              "mov ecx, %[destiny]\n\t"              "add ecx, %[i]\n\t"              "mov $[ecx], bl"              : [destiny] "=r" (destiny)              : [_source] "r" (_source), [i] "r" (i)              : "eax", "ecx", "bl"          );      }      _destiny = destiny;  }  

In the following code, I am getting two errors. "Operand Size Mismatch for 'mov'", and "No instruction mnemonic suffix given and no register operands; can't size instruction" My question is, what am I doing wrong and how can register values be used as memory addresses?

Edit 1: Code was changed to fit Intel Assembly Syntax

https://stackoverflow.com/questions/67255300/using-a-register-value-as-a-memory-address-in-gcc April 25, 2021 at 11:47PM