8086 architecture and programmer registers

 +---------------- BIU ----------------+
 |  CS   DS   SS   ES   (segment regs) |
 |  IP  (instruction pointer)          |
 |  Address adder  -> 20-bit address   |
 |  6-byte instruction queue           |
 |  Bus control logic                  |
 +------------------+------------------+
                    | internal bus
 +------------------+------------------+
 |  AX BX CX DX  (general registers)   |
 |  SP BP SI DI  (pointer/index)       |
 |  ALU      FLAG register             |
 |  Control unit + instruction decoder |
 +---------------- EU -----------------+
   BIU <-> external bus: AD0-AD15, A16-A19, control

The 8086 has two units: the BIU (fetches instructions, forms 20-bit addresses, handles the bus; contains segment registers, IP, address adder and the 6-byte queue) and the EU (decodes and executes; contains general registers, ALU, flags and control unit). They work in parallel.

Registers accessible to the programmer

1. General purpose registers (16-bit, splittable into 8-bit halves)

• AX (AH/AL), accumulator: arithmetic, logic, I/O (IN/OUT), MUL/DIV, string operations.
• BX (BH/BL), base: holds an offset address of data; used in XLAT.
• CX (CH/CL), count: loop counter for LOOP, count for shifts (CL) and REP.
• DX (DH/DL), data: I/O port address, high word of 32-bit multiply/divide result.

2. Pointer and index registers

• SP: stack pointer, offset of the top of stack (in SS).
• BP: base pointer, offset used to access stack-resident data/parameters.
• SI: source index for string and indexed addressing (DS:SI).
• DI: destination index (ES:DI for strings).

3. Segment registers

• CS: code segment base. DS: data segment base. SS: stack segment base. ES: extra segment base (string destination).

4. Instruction pointer (IP): offset of the next instruction within CS; not directly accessed but changed by jumps and calls.

5. Flag register: six status flags (CF, PF, AF, ZF, SF, OF) and three control flags (TF, IF, DF).