RTEMS 4.9.3 On-Line Library

Intel/AMD x86 Specific Information Flat Memory Model

RTEMS CPU Architecture Supplement

4.3.1: Flat Memory Model

RTEMS supports the i386 protected mode, flat memory model with paging disabled. In this mode, the i386 automatically converts every address from a logical to a physical address each time it is used. The i386 uses information provided in the segment registers and the Global Descriptor Table to convert these addresses. RTEMS assumes the existence of the following segments:

a single code segment at protection level (0) which contains all application and executive code.
a single data segment at protection level zero (0) which contains all application and executive data.

The i386 segment registers and associated selectors must be initialized when the initialize_executive directive is invoked. RTEMS treats the segment registers as system registers and does not modify or context switch them.

This i386 memory model supports a flat 32-bit address space with addresses ranging from 0x00000000 to 0xFFFFFFFF (4 gigabytes). Each address is represented by a 32-bit value and is byte addressable. The address may be used to reference a single byte, half-word (2-bytes), or word (4 bytes).

RTEMS CPU Architecture Supplement