These are instructions at the desired places in a program. The cs register is used for addressing a memory location in the code. Move cx into ds segment now we are in 2000 data segment. Can somebody explain how these registers are used, both, in user and kernelland programs. Offset memory address since all registers in the 8086 are 16 bits wide, the address space is limited to 216, or 65,536 64 k locations. All addresses are with reference to the segment registers. The 8086 has a segmented memory, the segment registers are used to manipulate memory within these segments.
Memory layout and access chapter four chapter one discussed the basic format for data in memory. Code, data, stack, extra segment registers in 8086. The 8086 is a 16bit architecture, with all internal registers 16 bits wide. Segmented memory will be discussed in more detail in section 1. The intel 80286 introduced a second version of segmentation in 1982 that added support for virtual memory and memory protection. If the stack is empty the stack pointer will be fffeh.
Each segment provides 6 4kb of memory, this area of memory is known as the current segment. In the x86 processor architecture, memory addresses are specified in two parts called the segment and the. Xchg ax, dx exchange word in ax with word in dx xchg bl, ch exchange byte in bl with byte in ch xchg al, prices bx exchange byte in al with byte in memory at. Written at the end of the procedure when it is executed, the previously stored. It decrements the stack pointer by 2 and pushes the. The 14 registers of 8086 microprocessor are categorized into four groups. There are instructions in 8086 which cause an interrupt. The 8086 provides four segment registers for address calculations. It disables the 8086 intr interupt input by clearing the interrupt flagif in the flag register. Cs the code segment register ds the data segment register ss the stack segment register es the extra segment register the biu fetches instructions using the cs and ip, written cs. Space the 808688 defines four 64kbyte memory blocks called the code segment, stack. The number of address lines in 8086 is 20, 8086 biu will send 20bit address, so as to access one of the 1mb memory locations. Segment registers hold the base address of where a particular segment begins in memory.
Segment size in x86 real mode, possibly a duplicate. Introduction of registers of 8086 microprocessor youtube. It contains all the type of branching instructions used in 8086 microprocessor. Memory segmentation the total memory size is divided into segments of various sizes.
The stack pointer and the stack segment register combine to form the complete. They are int instructions with type number specified. The 80286 microprocessor adds one major programmervisible feature to the 8086. The address line is 02 less than the earlier value. General purpose registers are used to store temporary data within the microprocessor. The segment registers cannot be used in this instruction. Memory segmentation in 8086 microprocessor geeksforgeeks. The registers ax, bx, cx, and dx are the general 16bit registers. No type register width name of the registers 1 general purpose registers4 16bit ax,bx,cx,dx 8bit al,ah,bl,bh,cl,ch,dl,dh. But if you are starting with 8086 its definitely going to be useful 8086 is a 16 bit microprocessor, so it has only 16 bit registers but it has 1 mb memory, so it needs 20 address lines 220. What are the advantages of using segment registers in the 8086 microprocessor.
Segment registers work together with general purpose register to access any memory value. There are four segment registers to access this 1 megabyte of memory. There are two types of registers in register set of 8086 microprocessor,they are 1general purpose registers and 2special purpose registers. The processor uses cs segment for all accesses to instructions. When one of these instructions is executed a branch to an iss takes place. Auxc denotes the auxiliary carry flag in the flags register. By combining the primitive string operations and iteration control operations with other. Description of general data registers,segment registers and pointers and index registers of intel 8086 microprocessor and brief introduction of flags. The 80868088 work differently from the later processors, but for all valid operands all 80x86. Every memory reference uses one of the four segment registers plus an offset andor a base pointer andor a index register. The ss stack segment register points at the segment containing the 8086 stack. Memory segmentation in memory, data is stored as bytes. Segment registers additional registers called segment registers generate memory address when combined with other in the microprocessor.
Segment registers are used to store the base address of the segment. Indeed, setting all segment registers to zero results in the previous 64k memory model. The bus interface unit biu contains four 16 bit special purpose registers mentioned below called as segment registers. This chapter presents the microprocessor as a programmable device by first. The first four registers are sometimes referred to as data registers. Data is fetched using a segment register usually the ds and an effective. If the bp register addresses memory, the stack segment is used by default. The segment register is multiplied by sixteen shifted to the left four bits and added to the sixteen bit result of the offset calculation. Intel 16bit hmos microprocessor specification sheet. What is the position of the stack pointer after the pop instruction. In the 64bit mode, the segment registers serve no purpose in addressing a location in the flat model.
Segmentation it is the process in which the main memory of computer is divided into different segments and each segment has its own base address. X86 assembly language programming for the pc 21 physical address the segment, pointer and other addressing registers are only 16 bits wide. Intel 8086 family users manual october 1979 author. Favourite answer normal address space in 8086 is 16 bit r which allows it to address 64k of linear memory space. On the other hand, the address put on the bus by the 8086 called physical address is 20bits. View and download intel 8086 specification sheet online. The data segment is used by default with register indirect addressing or any other mode that uses. How many types of registers are there in 8086 allinterview. The instructions that use specific registers include. Apr 09, 2020 memory segmentation of 8086 self help learning notes edurev is made by best teachers of self help learning. And yes 0xffffh is the highest possible offset for actual 8086, but a 386 in real mode can have higher segment limits aka unreal mode. Segment registers are 16bit registers which are assumed to be holding a 20 bit number, because it is assumed you will add a zero to the end.
What are the advantages of using segment registers in the. They came out with a chip the 8086 that featured some new registers called segment registers. It seems that this is heavily related to the segment registers %fs, %gs, %cs, %ss, %ds, %es. This document is highly rated by self help learning students and has been viewed 16 times. The only difference between an 8088 microprocessor and an 8086.
What is the role of segment register answer shabir gojree lpu. Some addressing modes combine more than one register and an offset value to. Eight of the registers are known as general purpose registers i. Figure 39 the 8086 divides its 1 mb of memory address space into four segments, the data, code, stack, and extra segments. The source may be any one of the segment register or other general purpose or special purpose registers or a memory location and another register or memory location may act as destination. Queue segment registers instruction pointer the queue if the biu is not busy. The es extra segment register is an extra segment register. Memory segmentation of 8086 self help learning notes edurev. In 8086 microprocessor, memory is divided into 4 segments as follow. Unit2 8086 assembly language programming ece department microprocessors and microcontrollers page 1 unitii 8086 assembly language programming.
The 8008 processor contains two register files and four 1bit flags. When combining two 32 bit registers in an addressing mode, the first register is the base. The process of dividing memory this way is called segmentation. I was reading in 8086 datasheet and i found this phrase that i couldnt understand in memory organisation. Segmentation was introduced on the intel 8086 in 1978 as a way to allow programs to address more than 64 kb 65,536 bytes of memory.
There are 8 general purpose registers in 8086 microprocessor. Offsets are combined with segment register contents to generate effective addresses. Note that the flags are upwardcompatible from the 80868088 to the pentium ii. Register organization of 8086 intel 8086 microprocessor.
It decrements the stack pointer by 2 and pushes the current code segment register contents on the stack 5. The technical term for that part of an address is the offset i. The new intel 8086 microprocessor was designed to provide an order of magnitude. Each segment provides 6 4kb of memory, this area of. Code segment cs is a 16bit register that is used for addressing memory location in the code segment of the memory 64kb, where the executable program is stored. I try to understand the process of memory segmentation for the i386 and amd64 architectures on linux. In intel microprocessor 8086, what is meant by segment.