Real Address Mode

In this mode (also referred to as Real Mode), the 386 behaves like a very fast Intel 8086 CPU with a few new instructions and wider registers. The accessible address space is officially limited to 1 megabyte, just like on the 8086 (although undocumented features of the 386 allow addressing 4GB even in real mode). For compatibility with earlier CPUs, the 386 starts operating in this mode after power on or reset.

In real mode, the processor calculates the physical address of a memory reference by shifting the value of a segment register to the left by 4 binary digits and then adding the offset address to this value. Thus, two 16-bit values (segment and offset) are combined to form a single 20-bit physical address. There are no linear addresses in real mode.

Under RTTarget-32, this mode is only used in the boot code. The boot code starts executing in real mode and carries out most of the initialization. Subsequently, it switches to 32-bit protected mode and never switches back.

Left The i386 Microprocessor

Down Virtual 8086 Mode