8086 Registers; Segment Registers and Physical Address; 8086 Flag Register

8086 Registers

24.1 General-Purpose Registers (16-bit, splittable into 8-bit halves)

16-bit	High 8	Low 8	Primary role
AX	AH	AL	Accumulator — arithmetic, I/O
BX	BH	BL	Base — base address in data segment
CX	CH	CL	Counter — loop counter (LOOP instruction)
DX	DH	DL	Data/I/O — I/O port address for IN/OUT

Valid pairs: AX, BX, CX, DX (each 16-bit as a whole).

24.2 Pointer and Index Registers

Register	Segment	Function
SP	SS	Stack Pointer — offset into stack segment
BP	SS	Base Pointer — offset for stack frame access
SI	DS (default)	Source Index — string/data source offset
DI	ES (default)	Destination Index — string/data destination offset

24.3 Instruction Pointer (IP)

16-bit offset into code segment
Points to next instruction to execute

Combined with CS to form physical address of instruction

Analogous to 8085 PC but segment-relative

24.4 Segment Registers

Register	Holds base of…
CS	Code Segment
DS	Data Segment
SS	Stack Segment
ES	Extra Segment

Each is 16 bits wide but represents the upper 16 bits of a 20-bit segment base (shifted left by 4).

Segment Registers and Physical Address

25.1 Segmented Memory Model

The 8086 divides the 1 MB address space into logical segments of up to 64 KB each (since offset is 16-bit: 0000H–FFFFH).

25.2 Physical Address Formula

Physical Address = (Segment Register × 10H) + Offset
 = (Segment Register × 16) + Offset

The segment value is shifted left 4 bits (one hex digit) and added to the 16-bit offset.

25.3 Worked Example 1 — Code Fetch

CS = 2000H, IP = 2345H

Physical Address = 2000H × 10H + 2345H
 = 20000H + 2345H
 = 22345H

25.4 Worked Example 2 — Stack Access

SS = 2450H, SP = 0024H

Physical Address = 2450H × 10H + 0024H
 = 24500H + 0024H
 = 24524H

25.5 Worked Example 3 — Data Access

DS = 3000H, BX = 0500H

Physical Address = 3000H × 10H + 0500H
 = 30000H + 0500H
 = 30500H

25.6 Segment Overlap

Because segment base is multiplied by 16 (not 64K), segments can overlap. Multiple segment:offset pairs can map to the same physical address:

1000H : 5000H → 10000H + 5000H = 15000H
1500H : 0000H → 15000H + 0000H = 15000H (same physical address!)

8086 Flag Register

The 8086 has 9 flags in a 16-bit flag register, divided into conditional flags (6) and control flags (3).

26.1 Conditional Flags (Status Flags)

Flag	Name	Set when…
CF	Carry	Carry/borrow out of MSB (unsigned overflow)
PF	Parity	Even parity in lower 8 bits of result
AF	Auxiliary Carry	Carry from bit 3 to bit 4 (BCD)
ZF	Zero	Result = 0
SF	Sign	MSB of result = 1
OF	Overflow	Signed overflow (result exceeds signed range)

8086 adds Overflow (OF) — not present in 8085's five flags.

26.2 Control Flags

Flag	Name	Function
TF	Trap	Single-step debugging (one instruction at a time)
IF	Interrupt Enable	Allow (IF=1) or block (IF=0) maskable interrupts
DF	Direction	String operation direction: 0 = forward (increment SI/DI), 1 = backward

26.3 8085 vs 8086 Flags Comparison

8085 (5 flags)	8086 equivalent	8086-only
S	SF

| Z | ZF | | |

| AC | AF | | |

| P | PF | | |

| CY | CF | | |

| — | — | OF, TF, IF, DF |