Real Address Mode

Real Address Mode

• Act as a fast 8086

• Instruction set is upwardly compatible

• It address only 1 M byte of physical memory using A0-A19.

•   In real addressing mode of operation of 80286, it just acts as a fast 8086. The instruction set is upward compatible with that of 8086.

The 80286 addresses only 1Mbytes of physical memory using A0- A19. The lines A20-A23 are not used by the internal circuit of 80286 in this mode. In real address mode, while addressing the physical memory, the 80286 uses BHE along with A0- A19. The 20-bit physical address is again formed in the same way as that in 8086.

The contents of segment registers are used as segment base addresses. The other registers, depending upon the addressing mode, contain the offset addresses. Because of extra pipelining and other circuit level improvements, in real address mode also, the 80286 operates at a much faster rate than 8086, although functionally they work in an identical fashion. As in 8086, the physical memory is organized in terms of segments of 64Kbyte maximum size.

An exception is generated, if the segment size limit is exceeded by the instruction or the data. The overlapping of physical memory segments is allowed to minimize the memory requirements for a task. The 80286 reserves two fixed areas of physical memory for system initialization and interrupt vector table. In the real mode the first 1Kbyte of memory starting from address 0000H to 003FFH is reserved for interrupt vector table. Also the addresses from FFFF0H to FFFFFH are reserved for system initialization.

The program execution starts from FFFFH after reset and initialization. The interrupt vector table of 80286 is organized in the same way as that of 8086. Some of the interrupt types are reserved for exceptions, single-stepping and processor extension segment overrun, etc.

When the 80286 is reset, it always starts the execution in real address mode. In real address mode, it performs the following functions: it initializes the IP and other registers of 80286, it prepares for entering the protected virtual address mode.