Appendix E — Memory internals

Purpose

Understand where program code and data live and why AVR's Harvard architecture affects programming.

Memory domains

• Flash: program instructions and persistent program constants.
• SRAM: runtime variables, .data, .bss, stack and optional heap.
• EEPROM: non-volatile data managed separately from normal SRAM.
• I/O/register space: control/status registers for CPU and peripherals.

Startup

Before main(), runtime startup normally establishes required CPU/runtime state, copies initialized data from its Flash image to SRAM and clears .bss.

Sections

Use the ELF/map/disassembly tools to locate .text, .data and .bss. Compare their sizes with actual MCU limits.

Stack

The stack grows and shrinks with calls, saved registers, local storage and interrupts. Measure stack movement in Exercise 04 and ISR exercises.

PROGMEM

Large constant tables need not consume scarce SRAM. Study the AVR distinction between ordinary data access and program-memory access before hiding it behind helper macros.

EEPROM

EEPROM is persistent but has different access semantics and write endurance. Exercise 11 provides the practical path.

Practice

For one firmware build, account for Flash and SRAM use, identify the stack start, and explain where every global/static object is initialized from and stored at runtime.