26-11-2012, 03:07 PM
80386 Real Mode
[attachment=41188]
Real mode programming Model
Initialise 80386 for the protected mode operation.
Shows only those parts of the 80386 which the programmer can access i.e. the registers within the microprocessor.
There are 16 bit and 32 bit registers. In the 16 bit segment registers are CS,DS,ES,SS,GS,FS and IP is also 16 bit. And 32 bit segment registers are EAX,EBX,ECX,EDX,ESP,EBP,ESI and EDI.
5 bit of the CRO are called as Machine Status Word (MSW).
Memory Addressing in Real Mode
80386 directly address up to 1 MB of memory. The address lines A2 - A19 ,BE0 – BE3 are used by the 80386 in the real mode.
A20 – A31 are normally high , but in case of intersegment JUMP and CALL ,this lines are low.
1 MB memory can’t active at one time so, it divided into 64 KB segment. Only six 64 KB can be active at a time.
Active segment –CS,DS,ES,SS,GS or FS.
Paging mechanism is not enabled so,the linear address are same as the physical address.
Only SI and DI can be used as index register and only BX and BP can be used as base register.
The Other general purpose register can not be used as address register. Scaling is also not supported.
[attachment=41188]
Real mode programming Model
Initialise 80386 for the protected mode operation.
Shows only those parts of the 80386 which the programmer can access i.e. the registers within the microprocessor.
There are 16 bit and 32 bit registers. In the 16 bit segment registers are CS,DS,ES,SS,GS,FS and IP is also 16 bit. And 32 bit segment registers are EAX,EBX,ECX,EDX,ESP,EBP,ESI and EDI.
5 bit of the CRO are called as Machine Status Word (MSW).
Memory Addressing in Real Mode
80386 directly address up to 1 MB of memory. The address lines A2 - A19 ,BE0 – BE3 are used by the 80386 in the real mode.
A20 – A31 are normally high , but in case of intersegment JUMP and CALL ,this lines are low.
1 MB memory can’t active at one time so, it divided into 64 KB segment. Only six 64 KB can be active at a time.
Active segment –CS,DS,ES,SS,GS or FS.
Paging mechanism is not enabled so,the linear address are same as the physical address.
Only SI and DI can be used as index register and only BX and BP can be used as base register.
The Other general purpose register can not be used as address register. Scaling is also not supported.