The 8086 addresses a segmented memory. The complete physical address which is 20-bits long is generated using segment and offset registers each of the size 16-bit.The content of a segment register also called as segment address, and content of an offset register also called as offset address. To get total physical address, put the lower nibble 0H to segment address and add offset address. The fig 1.3 shows formation of 20-bit physical address.
Register organization of 8086:
8086 has a powerful set of registers containing general purpose and special purpose registers. All the registers of 8086 are 16-bit registers. The general purpose registers, can be used either 8-bit registers or 16-bit registers. The general purpose registers are either used for holding the data, variables and intermediate results temporarily or for other purpose like counter or for storing offset address for some particular addressing modes etc. The special purpose registers are used as segment registers, pointers, index registers or as offset storage registers for particular addressing modes. Fig 1.4 shows register organization of 8086. We will categorize the register set into four groups as follows:
General data registers:
The registers AX, BX, CX, and DX are the general 16-bit registers.
AX Register: Accumulator register consists of two 8-bit registers AL and AH, which can be combined together and used as a 16- bit register AX. AL in this case contains the low-order byte of the word, and AH contains the high-order byte. Accumulator can be used for I/O operations, rotate and string manipulation.
BX Register: This register is mainly used as a base register. It holds the starting base location of a memory region within a data segment. It is used as offset storage for forming physical address in case of certain addressing mode.
CX Register: It is used as default counter or count register in case of string and loop instructions.
DX Register: Data register can be used as a port number in I/O operations and implicit operand or destination in case of few instructions. In integer 32-bit multiply and divide instruction the DX register contains high-order word of the initial or resulting number.
Segment registers:
To complete 1Mbyte memory is divided into 16 logical segments. The complete 1Mbyte memory segmentation is as shown in fig 1.5. Each segment contains 64Kbyte of memory. There are four segment registers.
Code segment (CS) is a 16-bit register containing address of 64 KB segment with processor instructions. The processor uses CS segment for all accesses to instructions referenced by instruction pointer (IP) register. CS register cannot be changed directly. The CS register is automatically updated during far jump, far call and far return instructions. It is used for addressing a memory location in the code segment of the memory, where the executable program is stored.
Stack segment (SS) is a 16-bit register containing address of 64KB segment with program stack. By default, the processor assumes that all data referenced by the stack pointer (SP) and base pointer (BP) registers is located in the stack segment. SS register can be changed directly using POP instruction. It is used for addressing stack segment of memory. The stack segment is that segment of memory, which is used to store stack data.
Data segment (DS) is a 16-bit register containing address of 64KB segment with program data. By default, the processor assumes that all data referenced by general registers (AX, BX, CX, DX) and index register (SI, DI) is located in the data segment. DS register can be changed directly using POP and LDS instructions. It points to the data segment memory where the data is resided.
Extra segment (ES) is a 16-bit register containing address of 64KB segment, usually with program data. By default, the processor assumes that the DI register references the ES segment in string manipulation instructions. ES register can be changed directly using POP and LES instructions. It also refers to segment which essentially is another data segment of the memory. It also contains data.
The pointers contain within the particular segments. The pointers IP, BP, SP usually contain offsets within the code, data and stack segments respectively
Stack Pointer (SP) is a 16-bit register pointing to program stack in stack segment.
Base Pointer (BP) is a 16-bit register pointing to data in stack segment. BP register is usually used for based, based indexed or register indirect addressing.
Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register indirect addressing, as well as a source data addresses in string manipulation instructions.
Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and register indirect addressing, as well as a destination data address in string manipulation instructions.
Flags Register determines the current state of the processor. They are modified automatically by CPU after mathematical operations, this allows to determine the type of the result, and to determine conditions to transfer control to other parts of the program. The 8086 flag register as shown in the fig 1.6. 8086 has 9 active flags and they are divided into two categories:
1. Conditional Flags
2. Control Flags
Conditional flags are as follows:
Carry Flag (CY): This flag indicates an overflow condition for unsigned integer arithmetic. It is also used in multiple-precision arithmetic.
Auxiliary Flag (AC): If an operation performed in ALU generates a carry/barrow from lower nibble (i.e. D0 – D3) to upper nibble (i.e. D4 – D7), the AC flag is set i.e. carry given by D3 bit to D4 is AC flag. This is not a general-purpose flag, it is used internally by the Processor to perform Binary to BCD conversion.
Parity Flag (PF): This flag is used to indicate the parity of result. If lower order 8-bits of the result contains even number of 1’s, the Parity Flag is set and for odd number of 1’s, the Parity flag is reset.
Zero Flag (ZF): It is set; if the result of arithmetic or logical operation is zero else it is reset.
Sign Flag (SF): In sign magnitude format the sign of number is indicated by MSB bit. If the result of operation is negative, sign flag is set.
Control flags are set or reset deliberately to control the operations of the execution unit. Control flags are as follows:
Trap Flag (TF): It is used for single step control. It allows user to execute one instruction of a program at a time for debugging. When trap flag is set, program can be run in single step mode.
Interrupt Flag (IF): It is an interrupt enable/disable flag. If it is set, the maskable interrupt of 8086 is enabled and if it is reset, the interrupt is disabled. It can be set by executing instruction sit and can be cleared by executing CLI instruction.
Direction Flag (DF): It is used in string operation. If it is set, string bytes are accessed from higher memory address to lower memory address. When it is reset, the string bytes are accessed from lower memory address to higher memory address.
