Extra segment register Example: code segment register CS points to a 64K region of memory: The extra segment register, es, is exactly that - an extra segment register. their base addresses (starting addresses) are stored in their respective segment registers. Extra Segment (ES): This register is an extra segment register that can be used to access additional data segments. Segment addresses combined with offset registers or instructions specify the physical memory location. Each segment is further divided into smaller units called paragraphs, each of which is 16 bytes in size. Although it is possible to store any data in the segment registers, this is never a good Oct 7, 2017 · Destination Index : The ES register points to the extra segment in which data is stored. SS - points at the segment containing the stack. In real mode, in order to calculate the As far as I understand, the 8086 processor has 4 memory segments: Stack Segment, Code Segment, Data Segment and Extra Segment, with the corresponding segment registers (SS,CS,DS,ES) The question i Oct 14, 2023 · We will also delve into related topics like the data segment, text segment, BSS segment, and extra segment, to provide a comprehensive understanding of how programs are loaded in memory and the relationship between these different segments. 8086 programs often use this segment register to gain access to segments when it is difficult or impossible to modify the other segment registers. Extra Segments (ES, FS, and GS): These extra segment registers point to different data sections. Another 16-bit register can act as an offset into a given segment, and so a logical address on this platform is written segment: offset, typically in hexadecimal notation. Only the microprocessor's compiler can do this. The operations of the SS are mainly Push and Pop. There are also Segment Register Combinations Code Segment – the CS register and IP (instruction pointer) are used to point to the next instruction. When using theflat (unsegmented) memory modelsegment This segment is used to hold the program to be executed. It is useful for storing temporary data or contents from other registers. DS - generally points at segment where variables are defined. SS : Holds the Stack segment your program uses. The document discusses memory segmentation in the Intel 8086 microprocessor, explaining how the memory is logically divided into segments (code, data, stack, and extra) to enhance execution speed. MOV cannot move segment register to a segment register. Mar 12, 2001 · In some processors with multiple base or segment registers, each base or segment register is used for different kinds of memory accesses (such as a segment register for data accesses and a different segment register for program accesses). Stack Segment (SS) Registers: The SS is used to store the information about the memory segment. Stack Segment (SS) Registers: The SS is used to store the Feb 3, 2021 · Segment selectors take the place of segment bases in segment register 80286. in 8086 microprocessor memory are divided into for parts which is known as the segments. There are four or more segment registers: CS contains the segment of the current instruction (IP is the offset), SS contains the stack segment (SP is the offset), DS is the segment used by default for most data operations, ES (and, in more recent processors, FS and GS) is an extra segment Jan 1, 2016 · SP - stack pointer. We can use them for anything! Memory addressing schemes: 1. If segments can be used to store extra data, how can I make sure that my storing data in them won't overwrite any existing data? For example, the CS register points to the Code Segment. ES - extra segment register, it’s up to a coder to define its usage. The visible of the segment register remains 16 bits long and holds a segment selector; the new hidden part, holding 48 bits, serve as a cache for a descriptor, which we discuss later. Extra segment (ES) It is a 16-bit register containing an address of 64KB segment, usually with program data. Special Purpose Registers The index and pointer registers are collectively called as special purpose registers. The data segment is the default source for string operations, but it can be overridden. The assembler will usually supply the override for you, but occasionally you must use an explicit segment override: mov WORD PTR es: [bx + 8 ], 100 You can call a procedure that is in a different segment. For example, if a program assigns a value to a variable, that data is stored in the data segment. DS (data segment), CS (code segment), SS (stack segment), and ES (extra segment). The Role of Segment Registers When the 8086 CPU needs to access memory, it utilizes the segment address of the memory unit, which is provided by the segment registers. Data Segment (DS) Register: The user can modify the content of the data segment. Segment registers point to a place in memory where one of the following things begin: Data storage Code execution. The user can modify the content of the data segment. The Code Segment contains the program's code. The extra segment register, es, is exactly that - an extra segment register. Feb 28, 2018 · By default, the processor assumes that all data referenced by general registers (AX, BX, CX, DX) and the index register (SI, DI) is located in the data segment. There are also The code segment register points to code instructions, data segment register points to global data, extra segment can access segments when other registers cannot change, and stack segment defines the memory area for the call stack. DS : Holds the Data segment that your program accesses. A 'Segment Register' in Computer Science is a memory area with 16 bits that can address up to 64 KB, used to define specific segments like code, data, stack, and extra segments in a computer's memory architecture. 64-bit x86 has additional registers. Oct 14, 2023 · We will also delve into related topics like the data segment, text segment, BSS segment, and extra segment, to provide a comprehensive understanding of how programs are loaded in memory and the relationship between these different segments. ES,FS,GS : These are extra segment registers available for far pointer addressing like video memory and such. Mar 24, 2012 · Memory Segmentation Model). Sep 6, 2020 · The es (Extra Segment) register is an extra segment register. By default, the processor assumes that the DI register references the ES segment in string manipulation instructions. In real mode, in order to calculate the As far as I understand, the 8086 processor has 4 memory segments: Stack Segment, Code Segment, Data Segment and Extra Segment, with the corresponding segment registers (SS,CS,DS,ES) The question i The default segment can be overridden by preceding the insturction by a segment override byte. The two words at offset 8 in the IVT (equal to linear address 00008h) are the offset and segment of the NMI (interrupt 2) handler. base MSRs with the addresses 0xC0000100 (for FS) and 0xC0000101 (for GS) contain the base addresses of the FS and GS segment registers. It can store the contents of other registers or offset addresses. Stack Segment Register (SS): is used for addressing stack segment of the memory. Here, it is important to note that these four memory segments are logical and do not physically divide in the memory. There are also Explore the various registers in the Intel x86 architecture, including general-purpose, segment, and control registers. 2. com Sep 15, 2025 · There are four primary segment registers: CS (Code Register), DS (Data Register), SS (Stack Register), and ES (Extra Segment Register). The four main segment registers are the code segment register (CS), data segment register (DS), extra segment register (ES), and stack segment register (SS). The Extra Segment (ES) register provides additional storage. Here is my question though. The extra segment register, es , is exactly that – an extra segment register. An Absolute Address , such as 04A26H, is a 20 bit value that directly references a specific location. How many segment registers are there? Dec 27, 2021 · The 8086 microprocessor has a powerful set of registers. To access data in one of the extra segments (es, fs, or gs ), only a single byte is neces- sary to choose the appropriate segment register. String instructions always use ES and DI to determined the 20-bit physical address for the destination. May 10, 2020 · ASM 8086 Cheat SheetPublished: 10th May, 2020 Last Updated: 13th May, 2020 CS : Holds the Code segment in which your program runs. DS register can be changed directly using POP and LDS instructions. The register organization of the 8086 microprocessor is also known as the programmer's model. Dec 29, 2018 · Using another segment register gives you the advantage of quick access to different segments - you are not limited to processing data only in one 64KB segment and do not have to change the DS register before each access to a different segment. 1 Segment-Register Transfer Instructions The MOV, POP, and PUSH instructions also serve to load and store segment registers. point to data in the machine language stack Segment Register CS - points at the segment containing the current program. The default segment can be overridden by preceding the insturction by a segment override byte. link- https://youtu. . The extra segment is the mandatory destination for string operations (for example MOVS or CMPS); for this one purpose only, the automatically selected segment register cannot be overridden. Jul 11, 2025 · Extra Segment Register (ES): also refers to a segment in the memory which is another data segment in the memory. The segment registers (CS, DS, SS, ES, FS, and GS) hold 16-bit segment selectors To access a particular segment in memory, the segment selector for that segment must be present in the appropriate segment register. Advantages include simplified memory access and organization, while a As far as i know, there's data segment, stack segment, extra segment and mentioned code segment? And since CS is "paired" with IP register, and uses it's 4 bits for offset, are other registers also paired with IP registers or each of these 4 segment registers has it's own offset register? Jan 13, 2025 · The Code Segment Register (CS) points to the start of this segment. Data Segment This segment is used to hold general data. Apr 26, 2024 · Segment values are stored in the segment registers. base, GS. these segments are data segment, code segment, stack segment and extra segment. The stack segment is that segment of memory which is used to store stack data. Segment and offset: Segments are special area defined in a program for containing the code, the data, and the stack. This segment also holds the source operands during string Jan 8, 2021 · This is a 2nd part of 8088cpu, if you want to understand full concept of register of 8088cpu you will have to watch 1st part also. Stack – the SS register is used with the SP (stack pointer) or BP (base pointer) Data Segment – DS with BX, SI, or DI Extended Segment – BX, SI, or DI Extra Segment (ES) − It is an additional data segment used for holding overflowed or auxiliary information. SEGMENT REGISTERS CS - points at the segment containing the current program. Mar 31, 2020 · Data Segment (DS) The Data Segment register is used to store data utilized by the program. In 80386 microprocessors, two additional segment registers were added which are Far Segment Register (FS) and Global Segment register (GS) which allows the memory access up to 6x64KB= 384K bytes. How segment registers are used depends on the type of memory management model that the operating system or executive is using. The Intel 8086 microprocessor has several registers that are categorized into general-purpose registers, segment registers, index registers, and control registers. For example, to access location 1234h in the extra segment (es) you would use an instruction of the form mov ax,es: [1234h]. Data Segment (DS) Register: Containing address of base location for variables. The processor fetches and runs instructions from here, either sequentially or based on the program’s flow. Segment registers specify the location of segments in memory and hold the starting addresses of different segments. A Segment Offset Address combines the starting address of a segment with an offset value. Only a few control transfer instructions allow you to specify a full 32 bit segmented address. Jul 23, 2025 · Extra Segment (ES): This register is used to hold the extra data by providing additional data segment in the memory. IP register (Instruction Pointer) holds the 16-bit offset address. Feb 10, 2021 · What is extra segment register? Extra Segment Register (ES): also refers to a segment in the memory which is another data segment in the memory. These are 16-bit registers used as memory pointers and These registers generates 20- bit physical address. Sometimes has the same value as Code Segment (CS) Register: Containing address of all executable instructions (code) in a program. This is known as a far call and both the original cs and ip pushed and popped. It highlights that the 16 segments are each 64KB, and the system utilizes 16-bit segment registers to access these segments. 8086 pro-grams often use this segment register to gain access to segments when it is difficult or impossible to modify the other segment registers. All other references to data use the data segment by default. Changing its value might give erronous data. Segment Offset within a program, all memory locations Oct 20, 2025 · FS. Extra Segment (ES) Register: Mar 31, 2020 · In this tutorial you will learn about various segment registers and their uses in Microprocessors. By default, all displacement-only values provide offsets into the data segment. CS register holds the 16-bit base address for this segment. So, let’s dive in and unravel the mysteries behind the code segment register in the 8086 microprocessor! Four registers are used to refer to four segments on the 16-bit x86 segmented memory architecture. Memory Segmentation and Physical address calculation Introduction Memory segmentation is nothing which is the methods where whole memory is divided into the smaller parts. 10. CS - points at the segment containing the current program. The data segment is used by most memory-access instructions, and the extra segment is used by the block operation instructions. Safe to contain anything, since use of a segment does not confer additional privileges to user code. Code Segment Stack Segmentmore Jan 1, 2024 · The x86 architecture has 8 General-Purpose Registers (GPR), 6 Segment Registers, 1 Flags Register and an Instruction Pointer. May 17, 2024 · Extra segment (ES) is a 16-bit register containing address of 64KB segment, usually with program data. If you want to provide an offset into a different segment, you must use a segment override prefix before your address. Segment registers are basically memory pointers located inside the CPU. There are code segment, stack segment, data segment and extra segment available in 8086 microprocessor. These and the DS register divide the program's memory into four distinct data sections. But the two bonus segment registers aren’t architecturally significant. Extra Segment (ES) The Extra Segment register serves as an additional data segment. Changing its value might make the computer hang. These variants operate similarly to their general-register counterparts except that one operand can be a segment register. Sep 16, 2020 · Probably es is loaded with the base value zero, to access the Interrupt Vector Table. See full list on tutorialspoint. The user cannot modify the content of these registers. ES - extra segment register, it's up to a coder to define its usage. Feb 4, 2019 · The code segment specifies which segment the instruction pointer is reading from. Segment Registers (CS, DS, SS and ES) The code segment register, data segment register, stack segment register, and the extra segment register are special registers related to the 3. b May 15, 2023 · Only the microprocessor's compiler can do this. These are commonly used for thread-pointers in user code and CPU-local pointers in kernel code. The ES register, referred to as the Extra Segment register, serves as an additional data segment. stos and movs write es:[di], which makes sense because DI is the "destination index" register. Instruction are fetched from the Code Segment. zishv tfqdx ptw vjxmzwa tdokh hpigmn rczfy pglh vrsowdx ykzzre etpxc ybj qhfoq lxnkbh jrogh