What Is The Name Of The Lowest 8 Bits Of The Edx Register?

Assembly - Registers

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Processor operations mostly involve processing data. This data tin can exist stored in memory and accessed from thereon. However, reading data from and storing data into retention slows down the processor, as it involves complicated processes of sending the data request across the control coach and into the memory storage unit and getting the information through the same aqueduct.

To speed up the processor operations, the processor includes some internal memory storage locations, called registers.

The registers store information elements for processing without having to access the retentiveness. A limited number of registers are built into the processor chip.

Processor Registers

There are x 32-flake and six 16-bit processor registers in IA-32 compages. The registers are grouped into three categories −

• General registers,
• Command registers, and
• Segment registers.

The general registers are further divided into the following groups −

• Data registers,
• Arrow registers, and
• Index registers.

Data Registers

Four 32-scrap data registers are used for arithmetic, logical, and other operations. These 32-bit registers can be used in three ways −

• As consummate 32-bit data registers: EAX, EBX, ECX, EDX.

• Lower halves of the 32-chip registers tin be used equally iv 16-flake data registers: AX, BX, CX and DX.

• Lower and higher halves of the in a higher place-mentioned four sixteen-bit registers tin exist used every bit 8 8-scrap data registers: AH, AL, BH, BL, CH, CL, DH, and DL.

Some of these information registers have specific use in arithmetical operations.

AX is the master accumulator; information technology is used in input/output and virtually arithmetic instructions. For case, in multiplication operation, one operand is stored in EAX or AX or AL register according to the size of the operand.

BX is known as the base of operations register, as it could be used in indexed addressing.

CX is known as the count register, as the ECX, CX registers shop the loop count in iterative operations.

DX is known as the data register. It is as well used in input/output operations. Information technology is also used with AX register forth with DX for multiply and split operations involving large values.

Arrow Registers

The pointer registers are 32-bit EIP, ESP, and EBP registers and respective xvi-scrap right portions IP, SP, and BP. At that place are three categories of pointer registers −

• Instruction Pointer (IP) − The 16-flake IP register stores the offset address of the side by side instruction to exist executed. IP in association with the CS register (as CS:IP) gives the complete address of the current instruction in the code segment.

• Stack Arrow (SP) − The sixteen-bit SP register provides the offset value inside the programme stack. SP in association with the SS register (SS:SP) refers to be current position of information or address within the plan stack.

• Base Arrow (BP) − The xvi-bit BP register mainly helps in referencing the parameter variables passed to a subroutine. The accost in SS register is combined with the offset in BP to get the location of the parameter. BP can as well be combined with DI and SI as base register for special addressing.

Index Registers

The 32-bit index registers, ESI and EDI, and their 16-bit rightmost portions. SI and DI, are used for indexed addressing and sometimes used in addition and subtraction. There are two sets of index pointers −

• Source Index (SI) − It is used as source index for string operations.

• Destination Index (DI) − It is used as destination index for string operations.

Control Registers

The 32-bit didactics pointer register and the 32-bit flags register combined are considered as the control registers.

Many instructions involve comparisons and mathematical calculations and alter the condition of the flags and some other conditional instructions test the value of these status flags to take the command flow to other location.

The common flag bits are:

• Overflow Flag (OF) − It indicates the overflow of a loftier-order scrap (leftmost bit) of data later a signed arithmetics performance.

• Direction Flag (DF) − It determines left or right direction for moving or comparing string data. When the DF value is 0, the string operation takes left-to-right direction and when the value is set to i, the cord functioning takes right-to-left direction.

• Interrupt Flag (IF) − It determines whether the external interrupts like keyboard entry, etc., are to be ignored or candy. It disables the external interrupt when the value is 0 and enables interrupts when fix to ane.

• Trap Flag (TF) − It allows setting the operation of the processor in single-step mode. The DEBUG program we used sets the trap flag, and then we could step through the execution one didactics at a time.

• Sign Flag (SF) − It shows the sign of the issue of an arithmetic operation. This flag is ready according to the sign of a information item following the arithmetic operation. The sign is indicated by the high-gild of leftmost fleck. A positive result clears the value of SF to 0 and negative upshot sets it to 1.

• Zip Flag (ZF) − It indicates the result of an arithmetic or comparison operation. A nonzero result clears the zero flag to 0, and a zero result sets information technology to i.

• Auxiliary Carry Flag (AF) − It contains the behave from bit iii to bit four following an arithmetics operation; used for specialized arithmetic. The AF is set when a 1-byte arithmetic functioning causes a carry from bit 3 into scrap 4.

• Parity Flag (PF) − Information technology indicates the total number of 1-bits in the result obtained from an arithmetic operation. An fifty-fifty number of 1-bits clears the parity flag to 0 and an odd number of 1-bits sets the parity flag to 1.

• Carry Flag (CF) − It contains the carry of 0 or 1 from a high-lodge bit (leftmost) afterward an arithmetic functioning. It also stores the contents of last bit of a shift or rotate operation.

The post-obit table indicates the position of flag bits in the 16-bit Flags annals:

Flag:					O	D	I	T	S	Z		A		P		C
Chip no:	15	14	13	12	11	10	9	8	7	vi	5	4	3	2	1	0

Segment Registers

Segments are specific areas defined in a program for containing data, lawmaking and stack. There are iii chief segments −

• Code Segment − It contains all the instructions to exist executed. A 16-fleck Code Segment register or CS annals stores the starting address of the code segment.

• Data Segment − It contains data, constants and work areas. A 16-bit Information Segment register or DS annals stores the starting address of the data segment.

• Stack Segment − Information technology contains data and return addresses of procedures or subroutines. Information technology is implemented as a 'stack' information structure. The Stack Segment register or SS annals stores the starting address of the stack.

Apart from the DS, CS and SS registers, there are other extra segment registers - ES (extra segment), FS and GS, which provide additional segments for storing information.

In assembly programming, a programme needs to admission the memory locations. All memory locations inside a segment are relative to the starting address of the segment. A segment begins in an accost evenly divisible past 16 or hexadecimal ten. So, the rightmost hex digit in all such retentivity addresses is 0, which is not more often than not stored in the segment registers.

The segment registers stores the starting addresses of a segment. To get the exact location of data or instruction within a segment, an first value (or displacement) is required. To reference whatsoever memory location in a segment, the processor combines the segment address in the segment register with the offset value of the location.

Example

Expect at the following simple program to understand the use of registers in assembly programming. This program displays 9 stars on the screen forth with a simple bulletin −

section	.text    global _start	 ;must exist declared for linker (gcc) 	 _start:	         ;tell linker entry indicate    mov	edx,len  ;bulletin length    mov	ecx,msg  ;message to write    mov	ebx,1    ;file descriptor (stdout)    mov	eax,4    ;system call number (sys_write)    int	0x80     ;call kernel 	    mov	edx,9    ;bulletin length    mov	ecx,s2   ;message to write    mov	ebx,1    ;file descriptor (stdout)    mov	eax,iv    ;system phone call number (sys_write)    int	0x80     ;telephone call kernel 	    mov	eax,1    ;system call number (sys_exit)    int	0x80     ;call kernel 	 department	.information msg db 'Displaying ix stars',0xa ;a message len equ $ - msg  ;length of bulletin s2 times 9 db '*'        

When the higher up code is compiled and executed, it produces the following issue −

Displaying 9 stars *********        

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Source: https://www.tutorialspoint.com/assembly_programming/assembly_registers.htm

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