You can access any location in memory. You need only to load its segment address and then refer to its offset address, using the appropriate segment register.
Because the offset address is 16 bits. This allows you to access 64kb with only one instruction.
The 8086/8088 is a 16 bit computer running on a 20 bit address bus. Processes use a segmented memory architecture to access one of four 64kb memory segments from a physical space of 1mb.
Near and far are obsolete terms used in the MSDOS and Windows 3.x platforms on the 8086/8088 processor. Near represents an area of memory that can be accessed using only a 16 bit offset and, as such, must lie within the default data segment, and is always less than 64kb in size. Far represents an area of memory that must be accessed using both a 16 bit offset and a 16 bit segment and, as such, can lie anywhere in memory and be larger than 64kb, at the expense of additional processing time and program size.
In x86 family of CPUs, there are six memory models. Memory is managed in "chunks" (segment or page) of 64K words. To find a specific location, the CPU needs to know the segment it is in and how far down (the offset) it is from the beginning of the segment. In addition, the x86 CPUs support three types of pointers: * near pointers access data in the specified segment * far pointers can access data within a 1MB address space * huge pointers are a type of far pointer, but with specialized behavior So applications written for the x86 world had to specify a type of memory model which detailed how pointers work. Generally speaking, the rules are: If code is under 64KB... and data is under 64KB... use Tiny or Small model If code is over 64KB... and data is under 64KB... use Medium model If code is under 64KB... and data is over 64KB... use Compact model If code is over 64KB... and data is over 64KB... use Large model
The 8086/8088 is a 16 bit processor running on a 16 bit (8086) or 8 bit (8088) bus with a 20 bit address bus. In order to obtain the extra 4 bits of addressibility, Intel designed segment registers that are effectively multiplied by four and then added to the 16 bit offset address generated by the instruction. This yields 64K segments of 64KB each, although they overlap each other at a distance of 16 bytes.
Segmented memory in the 8086 microprocessor allows for efficient organization and addressing of memory by dividing it into segments, such as code, data, and stack segments. This segmentation enables the CPU to access a larger memory space than what can be directly addressed by its 16-bit architecture, as each segment can be up to 64KB in size. It also facilitates memory protection and organization, allowing programs to manage different types of data and instructions more effectively. By using segment registers, the 8086 can quickly switch between different memory areas during program execution.
In an 8085 system, the memory word size required is 8 bits. This means that each memory location can store 8 bits or one byte of data. The 8085 processor accesses memory locations using these 8-bit memory addresses to read or write data during program execution. The memory word size of 8 bits allows the 8085 system to handle data in small, manageable chunks efficiently.
A 16-bit address bus can address 2^16 distinct memory locations, which equals 65,536 individual addresses. Since each address typically corresponds to one byte of memory, this allows for the addressing of up to 64 kilobytes (KB) of memory (64KB = 65,536 bytes). Thus, a 16-bit address bus can effectively access all memory within this range.
Virtual memory does not exist physically but it is available in the systemThe procedure of fetching the chosen pgm. Segments or data from the secondary storage into the physical memory is called swappingSo it can address 1GBPhysical address calculation in PVAMIt uses 16-bit content of a segment register as a selector to address a descriptor stored in physical memoryThe descriptor is a block of contiguous memory locations containing information of a segmentPrivilege levels prevent unauthorized accessesMaximum segment size will be of 64kb
The memory capacity of the 8085 microprocessor is 64 kb because the address bus is 16 bits, and you can address 216, or 64kb, with a 16 bit address bus.
The extra segment in the 8086/8088 is a 64kb region of memory that is indexed by the displacement address of the destination of certain string operations, relative to DI. Contrast that with the data segment, which is a 64kb region of memory that is indexed by the displacment address of the displacement of most operand addresses. The stack segment is similar, but it is used for stack oriented data, relative to SP or BP.
The 8086 can address 1,114,080 bytes. (One Mb + 64Kb - 16) That does not count I/O space, it only counts memory space.