Real, protected and virtual mode Segmentation = memory management scheme Segment = block of memory. A logical address consists of:
base address
offset (relative address within the segment) base address offset

segment

Advantages of the segmentation
shorter addressing part of the instruction (only the offset of the operand is encoded in the instruction, the base address is in a base register)
instructions are separated from data Segment types
code segment – holds machine instructions
data segment
stack segment – holds: o return addresses, parameters and local variables of procedures, o temporary results of mathematical operations Assembly Language 2/ 1

Real mode Processor 8086 – works only in real mode. 20-bit address bus => access up to 1,048,576 (or 220) memory locations (1 MB) Base address ... 16-bit value Offset ... 16-bit value => segments are limited to 216 bytes = 64 kB Linear address = base address * 16 + offset 15

0 base address 15

+

0000 0

offset

19

0 linear address

Linear address = physical address. What is the physical address, if the logical address is 020A:1BCD ?

Assembly Language 2/ 2

Protected mode – introduced in the 80386 processor. 32-bit address bus => access up to 232 bytes = 22 . 230 B = 4 GB Base address ... 32-bit value Offset ... 16-bit or 32-bit value Linear address = base address + offset Linear address → physical address. paging Paging – memory management scheme
transforms a huge logical address space to a smaller physical address space;
permits multitasking (execution multiple tasks in parallel) by providing tools for protection of the program’s address space (a program cannot overwrite other program’s data) Modes according to offset size:
16-bit mode (real or protected)
32-bit mode (only protected) Virtual mode Processor runs in protected mode, but simulates real mode: a 20-bit linear address is translated by paging to a 32-bit physical address. A processor is switched to virtual mode when running a DOS application under Windows operating system. Assembly Language 2/ 3

Registers – memory locations inside the processor.
user registers – used by user applications
system registers – used by the operating system

User registers
general-purpose registers
segment registers
instruction pointer
flags register

Assembly Language 2/ 4

General-purpose registers They may
hold instruction operands
contain the effective address (offset) of an operand or participate in its calculation. 31

16 15

EAX

8 7 AH AX

AL

EBX

BH

BX

BL

ECX

CH

CX

CL

EDX

DH

DX

DL

ESI

SI

EDI

DI

EBP

BP

ESP

SP

0

Assembly Language 2/ 5

Segment registers
16-bit registers
In real mode they contain the base address of a segment: CS – base address of the code segment SS – base address of the stack segment DS – base address of the data segment ES, FS, GS – base address of other data segments CS a SS are initialised automatically by the operating system at the moment of the program start. The other segment registers are set by program instructions.
In protected mode they contain indexes (selectors) to a table where segment descriptors are stored; they are initialised automatically by the operating system.

Assembly Language 2/ 6

Instruction Pointer EIP
32-bit register
It contains the memory offset at which the next instruction to be executed is stored.
In 16-bit mode the lower 16 bits of the EIP register (IP) are used: 1023:2AAA : inc cx 1023:2AAB : mov ax,Pocet CS

1023:2AAE : add ax,cx

EIP is initialised automatically by the operating system at the moment of the program start to the offset of the first instruction (usually 0). As one instruction is executed, EIP is advanced to point to the next instruction. Jumps, procedure calls and returns change the EIP value as well.

Assembly Language 2/ 7

Flags register
32-bit register
It contains information about – the result of the recent arithmetic or logical operation – the state of the processor – the state of the current task 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 0 0 0 0 0 0 0 0 0 0 RF 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 OF DF IF TF SF ZF 0 AF 0 PF 1 CF 0 used by the operating system

Assembly Language 2/ 8

Carry Flag CF CF is set to 1 if the result of an arithmetic operation on unsigned numbers is too big to be held in the specified register or memory location, i.e. the operation has produced a carry from the most significant bit. Otherwise it is set to 0. mov al,255; store 255 = 0FFh to register al add al,4; add 4 to al mov al,127 add al,4

Overflow Flag OF OF is set to 1 if an arithmetic operation on signed numbers gives a result which is out of range, i.e.
the operation has produced a carry to the most significant (sign) bit but not from the most significant bit,
the operation has produced a carry only from the most significant bit. mov al,-128; 80h add al,-1; 0FFh

Assembly Language 2/ 9

Sign Flag SF – is only useful when operating with signed numbers. SF is set to 1 if the result of an operation is negative, otherwise it is set to 0. SF takes the same value as the most significant bit of the result.

Auxiliary Carry Flag AF – is useful when operating with decimal numbers represented in packed BCD form. AF is set to 1 if an arithmetic operation has produced a carry from the 3rd to the 4th bit. mov al,28h add al,9 0010 1000 (= 28 BCD) 0000 1001 (= 09 BCD) 0011 0001 (= 31?)

Zero Flag ZF ZF is set to 1 if the result of an operation is zero, otherwise it is set to 0.

Assembly Language 2/ 10

Parity Flag PF PF is set to 1 if the lowest byte of the result of an operation contains an even number of 1s, otherwise it is set to 0. It is mainly useful when transmitting data between devices.

Direction Flag DF DF is useful when manipulating a data array (string, vector etc.). It is set by instructions in a program. If DF is set to 0, the string instruction increments the index by the size of the array entry and so progress is forward through memory (the string is manipulated from the left to the right). If DF is set to 1, the string instruction decrements the index and so progress is backward through memory (the string is manipulated from the end to the beginning).

Interrupt Enable Flag IF IF is set by instructions in a program. If IF is set to 1, interruption to normal processor operation can be initiated from devices (like keyboard, disc, modem). If IF is set to 0, external interrupts are ignored.

Assembly Language 2/ 11

Trap Flag TF Resume Flag RF These flags are useful when debugging programs. They are set by instructions in a program (debugger). By setting TF to 1 the processor is forced to operate in single step mode in which an internal interrupt is generated after every instruction. By setting RF to 1, a potential breakpoint on the next instruction will be ignored. Debugger sets RF to 1 after the breakpoint service routine has finished so the program can continue from the instruction labelled by breakpoint. After the instruction has been executed, RF is set to 0.

Assembly Language 2/ 12