Architecture

Introduction

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Ultibo core is, by design, intended to be a development environment for embedded devices which currently includes all models of the Raspberry Pi but this will most likely expand to include other popular development boards as time and resources permit. Because it is not an operating system and is not derived from an existing operating system, the design decisions made reflect the needs of the environment and intentionally leave out certain general purpose operating system concepts.

The unit of execution in Ultibo core is the thread, there is no concept of a process but multi threading is fundamental to the design and cannot be removed. There is also no concept of a user space or kernel space, all threads run within a single address space and all threads have equal privileges.

All elements are intended to be dynamic in their implementation, for example devices are intended to be added and removed dynamically with notifications used to signal interested parties about the addition or removal. Only a few internal items have a fixed limit, a specific example being TLS index slots which are statically allocated during boot.

The design is also intended to be modular so that only the required elements are included in any given application, a small number of mandatory elements are included in every application such as memory management, threading and scheduling, interrupt handling, device management and the core functionality of the FPC run time library. Most other features such as the filesystem and network interfaces, USB, MMC/SD and hardware specific drivers can be omitted if they are not required.

Platform support

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With an intention to support a range of embedded devices and development boards over time, the architecture of Ultibo core tries to separate platform specific functionality from common functionality. Each supported board has both a boot module and a platform module which are specific to that board and provide the necessary initialization and platform functionality to support the common modules without requiring conditional compilation.

For the Raspberry Pi (Models A, B, A+, B+ and Zero*) the board specific modules are:

BootRPi.pas

PlatformRPi.pas

* Each of these models use the BCM2835 System on Chip which means they are identical from a code perspective, differences can be detected by model or revision numbers.

For the Raspberry Pi 2B the board specific modules are:

BootRPi2.pas

PlatformRPi2.pas

In addition there are architecture specific modules for the ARM processor and for the ARM processor type, these include:

PlatformARM.pas (for functionality common to all ARM processors)

PlatformARMv6.pas (this includes the ARM1176JZF-S used in the Raspberry Pi A, B, A+, B+ and Zero)

PlatformARMv7.pas (this includes the ARM Cortex-A7 MPCore used in the Raspberry Pi 2B

The appropriate boot module is included in an application based on the Controller Type selected and passed to the compiler (-Wp), the boot module then includes the relevant platform and architecture modules as well as the mandatory common modules for platform, memory, threads and devices etc.

Memory Management

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The HeapManager unit is a mandatory inclusion in all applications, during boot it registers itself with the RTL so that memory allocation via GetMem, FreeMem etc function normally. The platform specific module for the board then registers with the HeapManager all of the available blocks of memory that can be allocated, how this occurs is specific to the board but on the Raspberry Pi it is passed by the boot loader in a tag (or ATAG) parameter.

All memory allocation must go via the HeapManager either by calling the standard RTL functions or by calling the additional functions it provides, depending on the platform, the command line and information passed by the boot loader various types of memory are registered and made available for allocation. These include Normal, Shared, Non Cached, Code and Device memory which are required by certain Ultibo core components or can be requested by calling HeapManager functions.

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