A PC has an operating system (Windows or Linux) and this runs programs, such as Word or Internet Explorer or Chrome that do specific things. An 8-bit microcontroller like the AVR doesn’t usually have an operating system, although it could run a simple one if required, and instead it just runs a single program.
AVR is a family of microcontrollers developed by Atmel beginning in 1996. These are modified Harvard architecture 8-bit RISC single-chip microcontrollers. AVR was one of the first microcontroller families to use on-chip flash memory for program storage, as opposed to one-time programmable ROM, EPROM, or EEPROM used by other microcontrollers at the time. AVR microcontrollers find many applications as embedded systems; they are also used in the Arduino line of open source board designs.
The AVR's internal EEPROM is accessed via special registers inside the AVR, which control the address to be written to (EEPROM uses byte addressing), the data to be written (or the data which has been read) as well as the flags to instruct the EEPROM controller to perform a write or a read.
Flash Program memory can be reprogrammed In-System through an SPI serial interface, by a conventional non-volatile memory programmer, or by an On-chip boot program running on the AVR core. The boot program can use any interface to download the application program in the
To copy a value to a memory location in SRAM you have to define the address. The SRAM addresses you can use reach from 0x0060 (hex notation) to the end of the physical SRAM on the chip (in the AT90S8515 the highest accessable internal SRAM location is 0x025F).
The timer and counter functions in the microcontroller simply count in sync with the microcontroller clock. However, the counter can only count up to either 256 (8-bit counter), or 65535 (16-bit counter). That's far from the 1,000,000 ticks per second that the standard AVR microcontroller provides.