Memory Layout

This document describes how the memory in Tock is structured and used for the kernel, applications, and supporting state.

• Flash
  • Kernel code
  • Process code
• RAM
  • Kernel RAM
  • Process RAM
• Hardware Implementations
  • SAM4L
    • Flash
    • RAM
    • Overview

Tock is intended to run on microcontrollers like the Cortex-M, which have non-volatile flash memory (for code) and RAM (for stack and data) in a single address space. While the Cortex-M architecture specifies a high-level layout of the address space, the exact layout of Tock can differ from board to board. Most boards simply define the beginning and end of flash and SRAM in their layout.ld file and then include the generic Tock memory map.

Flash

The nonvolatile flash memory holds the kernel code and a linked-list of sorts of process code.

Kernel code

The kernel code is split into two major regions. The first is .text, which holds the vector table, program code, initialization routines, and other read-only data. This section is written to the beginning of flash.

The second major region following up the .text region is the .relocate region. It holds values that need to exist in SRAM, but have non-zero initial values that Tock copies from flash to SRAM as part of its initialization (see Startup).

Process code

Processes are placed in flash starting at a known address which can be retrieved in the kernel using the symbol _sapps. Each process starts with a Tock Binary Format (TBF) header and then the actual application binary. Processes are placed continuously in flash, and each process's TBF header includes the entire size of the process in flash. This creates a linked-list structure that the kernel uses to traverse apps. The end of the valid processes are denoted by an invalid TBF header. Typically the flash page after the last valid process is set to all 0x00 or 0xFF.

RAM

The RAM holds the data currently being used by both the kernel and processes.

Kernel RAM

The kernel RAM contains three major regions:

1. Kernel stack.
2. Kernel data: initialized memory, copied from flash at boot.
3. Kernel BSS: uninitialized memory, zeroed at boot.

Process RAM

The process RAM is memory space divided between all running apps.

A process's RAM contains four major regions:

1. Process stack
2. Process data
3. Process heap
4. Grant

The figure below shows the memory space of one process.

Hardware Implementations

SAM4L

The SAM4L is a microcontroller used on the Hail and Imix platforms, among others. The structure of its flash and RAM is as follows.

Flash

Address Range	Length (bytes)	Content	Description
0x0-3FF	1024	Bootloader	Reserved flash for the bootloader. Likely the vector table.
0x400-0x5FF	512	Flags	Reserved space for flags. If the bootloader is present, the first 14 bytes are “TOCKBOOTLOADER”.
0x600-0x9FF	1024	Attributes	Up to 16 key-value pairs of attributes that describe the board and the software running on it.
0xA00-0xFFFF	61.5k	Bootloader	The software bootloader provides non-JTAG methods of programming the kernel and applications.
0x10000-0x2FFFF	128k	Kernel	Flash space for the kernel.
0x30000-0x7FFFF	320k	Apps	Flash space for applications.

RAM

Address Range	Length (bytes)	Content	Description
0x20000000-0x2000FFFF	64k	Kernel and app RAM	The kernel links with all of the RAM, and then allocates a buffer internally for application use.

Overview

The following image gives an example of how things are currently laid out in practice. It shows the address space of both flash and RAM with three running applications: crc, ip_sense, and analog_comparator.