x86 16-Bit Real Mode Cheat Sheet

ModR/M Byte

8-bit follower for variable-length addressing (after opcode).

Specifies op mode (reg vs. mem), registers, and effective address (EA).

Bit Structure:

7 6	5 4 3	2 1 0
mod	reg	r/m

• mod (7-6): Addressing mode.
• reg (5-3): Register operand.
• r/m (2-0): Register or memory mode (EA calc).

Segment Registers

Overview

• Mode: 16-bit real mode (8086/8088, emulated in later x86).
• Memory Model: Segmented addressing; 1 MB total (2^20 bytes), 64 KB segments (16-byte aligned).
• Address Calculation: Physical = (Segment × 16) + Offset (both 16-bit values).
• Segments: Overlapping possible; no protection (unlike protected mode).
• Registers: 4 segment registers (CS, DS, SS, ES); loaded via specific instructions (e.g., MOV).

Memory Addressing

• Segment: Base address = segment << 4 (or segment * 16).
• Offset: 0x00000xFFFF (64 KB max).
• Defaults:
  • Code: CS:IP
  • Stack: SS:SP
  • Data: DS:BX/SI/DI; ES:DI (strings).
• Overrides: Prefixes like CS:, DS:, etc.
  • MOV AX, CS:[BX] / movl %cs:(%bx), %eax

Segment Registers Table

Register	reg bits	Purpose	Default Usage	Load Notes
ES - Extra	000	Auxiliary data (e.g., strings)	String operations	MOV ES, reg / movw %reg, %es; no core default.
CS - Code	001	Executable code segment	JMP/CALL (far); code fetch	Indirect (via JMP/CALL only).
SS - Stack	010	Stack (calls, interrupts, locals)	PUSH/POP/CALL/RET	MOV SS, reg / movw %reg, %ss; no immediate IP change.
DS - Data	011	Global/static data (vars)	General data ops	MOV DS, reg / movw %reg, %ds; override w/ prefixes.

Limitations: No FS/GS (added in 32-bit); segments wrap/overlap.

ModR/M Byte for Segments

• reg (5-3): Segment encoding (for MOV Sreg, r/m / MOV r/m, Sreg).
• r/m (2-0): Other operand (GPR or mem mode).

Relevant Opcodes are Only

• 0x8E MOV Sreg, r/m
• 0x8C MOV r/m, Sreg

General-Purpose Registers (GPRs)

8 registers (16-bit each); used for arithmetic, data movement, addressing.

ModR/M Role: Encodes GPRs in reg and r/m fields

• reg: Destination/source register
• r/m: Source/destination register/memory-mode

GPR Encoding Table

Register	Full Name	8-Bit Low/High	reg / r/m	Order	Common Use
AX	Accumulator X	AL/AH	000	0	Math, I/O, MUL/DIV.
CX	Counter X	CL/CH	001	1	Loops (LOOP), shifts.
DX	Data X	DL/DH	010	2	I/O, MUL/DIV high.
BX	Base X	BL/BH	011	3	Base addressing.
SP	Stack Pointer	(N/A)	100	4	Stack ops (PUSH/POP).
BP	Base Pointer	(N/A)	101	5	Stack frames.
SI	Source Index	(N/A)	110	6	String source.
DI	Dest Index	(N/A)	111	7	String dest.

Notes:

• [Disp] = absolute offset (DS:Disp).
• When r/m=101 and mod=00, the effective address is [BP] (not [DI]). BP and [BP] modes use SS by default; override with segment prefixes.
• Displacement modes (mod=01/10) add 8-bit or 16-bit signed offsets to the base address.

Tips

• Defaults: Many ops use AX implicitly (e.g., MUL AX).
• Index Regs: SI/DI auto-increment in strings (e.g., MOVSB).
• Refs: Avoid direct SP/BP in some arith (flags issues).

8-Bit Register Encoding

Some 8 bit instructions have a base primary opcode which an offset must be added to in order to select a different register.

Register	Byte Type	Decimal Offset	3-Bit Value (Binary)
AL	Low	0	000
CL	Low	1	001
DL	Low	2	010
BL	Low	3	011
AH	High	4	100
CH	High	5	101
DH	High	6	110
BH	High	7	111

Example: MOV AH, 0x42 offset 4, opcode 0xB4, encoding: B4 42 (high byte of AX only).

Note: Only applicable to the GPRs and not the Index Registers di and si.

ModR/M Memory Modes

Memory Modes: When mod is 11, computes EA = Segment:Offset (defaults to DS).

Real Mode: 16-bit addresses; disp 8/16-bit; no scaling (unlike 32-bit).

Mod Field (Addressing Modes)

mod bits	Disp Bytes	Mode Description
00	0	No displacement; direct mem or reg-indirect.
01	1	8-bit signed displacement + indirect.
10	2	16-bit signed displacement + indirect.

Reg and R/M Fields

The ModR/M byte contains:

• reg: Selects a register operand (or opcode extension for single-operand instructions).
• r/m: Selects the other operand: a register (mod=11) or memory address (mod=00/01/10).

Memory Addressing (mod=00/01/10)

The r/m field selects from 8 predefined addressing modes:

r/m	Base	Index	Default Segment
000	BX	SI	DS
001	BX	DI	DS
010	BP	SI	SS
011	BP	DI	SS
100		SI	DS
101		DI	DS
110	BP++		SS
111	BX		DS

++ - if mod is 00, direct addressing with 2 displacement bytes, no Base, uses DS segment

Segment Override: Preceed the opcode to override the default segment register

Opcode Prefix	Segment Override
26	ES
2E	CS
36	SS
3E	DS

x86 16-bit Real Mode: BIOS Handoff State to MBR (at 0x7C00)

Upon BIOS completion (POST, boot device scan), the 512-byte MBR sector is loaded to physical address 0x7C00 and verified (signature 0xAA55 at bytes 510-511). Control transfers via a far jump (e.g., JMP 0x0000:0x7C00 or JMP 0x07C0:0x0000), setting the effective address to 0x7C00. Registers are not standardizedassume undefined/random except DL. Interrupts are typically enabled (IF=1), so use CLI immediately. The bootloader must initialize segments and stack explicitly.

Register	BIOS Handoff Value	Notes
IP	0x7C00 or 0x0000	From far jump to 0x7C00; e.g., IP=0x7C00 with CS=0x0000, or IP=0x0000 with CS=0x07C0. Normalize with far jump in MBR if needed.
CS	0x0000 or 0x07C0	Adjusted by BIOS to point to 0x7C00; variations existset explicitly to 0x0000 in MBR for consistency.
DS, ES,	Undefined (often 0x0000)	Not guaranteed; may be non-zeroset to 0x0000 explicitly.
SS	Undefined (often 0x0000)	Set explicitly (e.g., SS=0x0000); unsafe until SP is also set.
SP	Undefined/random	Often near/below 0x7C00; set explicitly before stack use (e.g., SP=0x7BFF for safety, or SP=0x9000 for more space). Stack grows downward from SP.
AX, BX, CX, DX (excl. DL) SI, DI, BP	Undefined/random	Initialize as needed; no assumptions.
DL	Boot device number (e.g., 0x80 for HDD0)	Generally reliable (99%+ cases), but rare BIOS bugs may occur (e.g., 0x00)save immediately (e.g., PUSH DX) for INT 0x13.
FLAGS	Varies (IF=1 typically)	Interrupts enabled by BIOS; use CLI immediately, then STI after initialization. Other flags (CF, ZF, DF, etc.) are undefined/random.

IBM PC VGA Text Mode Memory Cheat Sheet

Video Memory Location

Property	Value
Base Address	0xB8000 (real mode)
Screen Size	80 columns × 25 rows (standard)
Bytes per Character	2

Memory Offset Formula:

offset = (row * 80 + column) * 2

Character Memory Structure

Each character position requires 2 consecutive bytes:

1. CHARACTER BYTE - Contains ASCII code of character to display. Range: 0x00-0xFF (0-255). Example: 0x41 = 'A', 0x30 = '0', 0x20 = space
2. ATTRIBUTE BYTE - Controls foreground color, intensity, background color, and blink

Endian Note: When writing machine code ensure to swap the order for little endian.

Attribute Byte Bit Layout

Bit Layout:

7	6	5	4	3	2	1	0
BL	BG2	BG1	BG0	I	FG2	FG1	FG0

Bits	Name	Description
0-2	FG2-FG0	Foreground color (0-7)
3	I	Foreground intensity (0=normal, 1=bright)
4-6	BG2-BG0	Background color (0-7)
7	B	Blink flag (0=normal, 1=blink)

Color Palette (8 Colors)

Value	Name	Value	Name	Value	Name	Value	Name
0	Black	1	Blue	2	Green	3	Cyan
4	Red	5	Magenta	6	Yellow	7	White

INTENSITY: Add 0x08 to base color value for bright version

Example: 0x02 = Dark Green, 0x0A = Bright Green

Common Attribute Byte Examples

Decimal	Hex	Foreground Color (bright)	Background Color
7	0x07	White (normal)	Black
15	0x0F	White (bright)	Black
10	0x0A	Green (bright)	Black
12	0x0C	Red (bright)	Black
14	0x0E	Yellow (bright)	Black
11	0x0B	Cyan (bright)	Black
13	0x0D	Magenta (bright)	Black
9	0x09	Blue (bright)	Black
32	0x20	Black (normal)	Green
64	0x40	Black (normal)	Red
96	0x60	Black (normal)	Yellow
112	0x70	Black (normal)	White
135	0x87	White (bright) + BLINK	Black

Attribute Byte Calculator

Formula: (background << 4) | (intensity << 3) | foreground

Example: Bright green foreground on red background

background = 4 (red)
intensity = 1 (bright)
foreground = 2 (green)

Result = (4 << 4) | (1 << 3) | 2 = 0x4A = 74 decimal

Quick Reference Table

To create attribute byte for "Text on Background":

FG / BG	BLACK	BLUE	GREEN	CYAN	RED	MAGENTA	YELLOW	WHITE
BLACK	0x00	0x10	0x20	0x30	0x40	0x50	0x60	0x70
BLUE	0x01	0x11	0x21	0x31	0x41	0x51	0x61	0x71
GREEN	0x02	0x12	0x22	0x32	0x42	0x52	0x62	0x72
CYAN	0x03	0x13	0x23	0x33	0x43	0x53	0x63	0x73
RED	0x04	0x14	0x24	0x34	0x44	0x54	0x64	0x74
MAGENTA	0x05	0x15	0x25	0x35	0x45	0x55	0x65	0x75
YELLOW	0x06	0x16	0x26	0x36	0x46	0x56	0x66	0x76
WHITE	0x07	0x17	0x27	0x37	0x47	0x57	0x67	0x77

Notes:

• Add 0x08 to foreground color value for bright intensity
• Add 0x80 to enable blinking

Practical Examples

Example 1: Bright White 'A' on Black Background

Component	Value	Description
Character Byte	0x41	ASCII 'A'
Attribute Byte	0x0F	Bright white on black
Memory	[0x41, 0x0F]

Example 2: Blinking Red 'X' on Yellow Background

Component	Value	Description
Character Byte	0x58	ASCII 'X'
Attribute Byte	0xEC	Red (bright) on yellow + blink
Memory	[0x58, 0xEC]

Example 3: Dark Blue '0' on White Background

Component	Value	Description
Character Byte	0x30	ASCII '0'
Attribute Byte	0x71	Dark blue on white
Memory	[0x30, 0x71]

Memory Writing In Assembly

Example 1: Write bright green 'G' at top-left (0,0)

Intel Syntax (IBM): mov ax, 0xB800 ; Video memory segment mov es, ax xor di, di ; Offset 0 (top-left) mov al, 'G' ; Character mov ah, 0x0A ; Bright green on black mov word [es:di], ax

AT&T Syntax: movw $0xB800, %ax # Video memory segment movw %ax, %es xorw %di, %di # Offset 0 (top-left) movb $'G', %al # Character movb $0x0A, %ah # Bright green on black movw %ax, %es:(%di)

Example 2: Move to position (10, 5)

Intel Syntax (IBM): ; Calculate offset: (5 * 80 + 10) * 2 = 820 mov di, (5 * 80 + 10) * 2 mov word [es:di], ax

AT&T Syntax: ## Calculate offset: (5 * 80 + 10) * 2 = 820 movw $(5 * 80 + 10) * 2, %di movw %ax, %es:(%di)

Example 3: Write string "Hello" starting at (0, 0)

Intel Syntax (IBM): mov ax, 0xB800 ; Video memory segment mov es, ax xor di, di ; Offset 0 (top-left) mov ah, 0x0F ; Bright white on black mov al, 'H' mov word [es:di], ax add di, 2 mov al, 'e' mov word [es:di], ax add di, 2 mov al, 'l' mov word [es:di], ax add di, 2 mov al, 'l' mov word [es:di], ax add di, 2 mov al, 'o' mov word [es:di], ax

AT&T Syntax: movw $0xB800, %ax # Video memory segment movw %ax, %es xorw %di, %di # Offset 0 (top-left) movb $0x0F, %ah # Bright white on black movb $'H', %al movw %ax, %es:(%di) addw $2, %di movb $'e', %al movw %ax, %es:(%di) addw $2, %di movb $'l', %al movw %ax, %es:(%di) addw $2, %di movb $'l', %al movw %ax, %es:(%di) addw $2, %di movb $'o', %al movw %ax, %es:(%di)

Tips

• Video memory address starts at 0xB8000
• Each character needs 2 bytes: [ASCII][ATTRIBUTE]
• Use offset formula: (row * 80 + col) * 2
• Foreground colors: bits 0-2, add 0x08 for bright
• Background colors: bits 4-6 (no intensity bit)
• Bit 7 enables blinking
• Standard screen is 80×25 characters
• Memory is directly writable in real mode or protected mode

BIOS Interrupt Services Cheat Sheet

Check the carry flag (CF) for errors on I/O ops (CF=0 success). Data from standard references like Ralf Brown's Interrupt List (RBIL) and IBM PC technical docs.

Boot-time Register States: At bootloader entry (after BIOS loads sector to 0x7C00): DL=boot drive number (00h=floppy, 80h=first HDD, 81h=second HDD, etc.). CS:IP typically 0000h:7C00h. Other registers undefinedinitialize as needed.

Interrupts are invoked with INT <number>.

Preserve registers state with stack (don't forget to initialize) if needed before interrupt instruction.

Availability varies by BIOS version/hardware (e.g., AT vs. XT) so see compatibility table in next section.

INT 10h: Video Services

Controls display modes, text/graphics output, and cursor. Most useful for early console I/O. Includes VESA BIOS Extensions (VBE) for modern graphics.

AH	AL/Other	Function	Inputs	Outputs	Notes
00h	AL=mode	Set video mode	AL=mode (e.g., 00h=40x25 text, 13h=320x200x256)	None	Clears screen; modes 013h common.
01h	CH=start, CL=end	Set cursor size	CH=scanline start (031), CL=end (start31)	None	Bits 57 of CH for cursor options (e.g., insert).
02h	BH=page, DL=col, DH=row	Set cursor position	BH=page (07), DX=position	None	Default page 0.
03h	BH=page	Get cursor position	BH=page	DH=row, DL=col, CH=start, CL=end	Returns current cursor.
05h	AL=page	Select active display page	AL=page (07)	None	For text modes only.
06h	AL=lines, BH=attr, CX=upper-left, DX=lower-right	Scroll up	AL=lines to scroll (0=clear), BH=fill attr	None	CX/DX=coords (CH=upper row, CL=left col, etc.).
07h	Same as 06h	Scroll down	Same as 06h	None	Negative scroll effect.
08h	BH=page	Read char/attr at cursor	BH=page	AL=char, AH=attr	Text modes.
09h	AL=char, BH=page, BL=attr, CX=count	Write char + attr	As named	None	No cursor advance.
0Ah	AL=char, BH=page, BL=attr, CX=count	Write char only	As named	None	Attr unchanged.
0Bh	BH=page, BL=color	Set palette	BL=0 (norm), 1 (intens)	None	Sub: 00h=set norm, 01h=set intens.
0Ch	AL=color, BH=page, CX=x, DX=y	Write pixel	AL=color, BH=page, CX=column (x), DX=row (y) (graphics)	None	Modes 46,1113.
0Dh	BH=page, CX=x, DX=y	Read pixel	BH=page, CX=column (x), DX=row (y)	AL=color	Graphics modes.
0Eh	AL=char	Teletype output	AL=char (ASCII)	None	Advances cursor; handles scroll. Best for simple print.
0Fh	BH=page	Get video mode	BH=page	AL=mode, AH=columns	-
10h	-	Palette functions	See sub (e.g., AH=10h, AL=00h=load DAC)	Varies	Advanced: e.g., 01h=select palette.
11h	-	Font loading	E.g., AL=01h=8x8 font load	Varies	For custom chars.
12h	-	Alternate functions	E.g., AL=10h=EGA info	Varies	Hardware-specific.
13h	ES:BP=string, CX=bytes, AL=mode, BH=page, BL=attr, DX=pos	Write string	As named (mode 0=attr, 1=no attr, 2=repeat)	None	AT-compatible; efficient for text.
4Fh	00h	Get VBE info	ES:DI=VbeInfoBlock (512 bytes)	AL=4Fh, AH=status (00h=success)	Returns VESA capabilities.
4Fh	01h	Get mode info	CX=mode number, ES:DI=ModeInfoBlock (256 bytes)	AL=4Fh, AH=status	Mode details.
4Fh	02h	Set VBE mode	BX=mode (bit 15=1: linear framebuffer, bit 14=1: preserve memory)	AL=4Fh, AH=status	Modern graphics modes.

INT 11h: Equipment Configuration

AH	Function	Inputs	Outputs	Notes
-	Get equipment list	None	AX=flags (bit0=floppy,1=math copro,23=video,45=drives,6=keyboard,7=printer,1415=RAM pages)	Single call; e.g., bits 45 = #floppy drives (03).

INT 12h: Memory Size

AH	Function	Inputs	Outputs	Notes
-	Get conventional memory	None	AX=KB (up to 640)	Quick; for stack/heap sizing.

INT 13h: Disk Services

Low-level block I/O (CHS or LBA). DL=drive (0x00=floppy0, 0x80=HDD0). Reset with 00h before ops. CH=low 8 bits of cylinder (01023); CL bits 0-5=sector (1-63), bits 6-7=high 2 bits of cylinder; DH=head (0255).

AH	Function	Inputs	Outputs	Notes
00h	Reset disk	DL=drive	CF=0 success	Clears errors.
01h	Get status	DL=drive	AH=status (0=OK,1=invalid), CF=1 error	Post-read/write check.
02h	Read sectors	AL=#secs (1128), CH/DH/CL=loc, ES:BX=buf	CF=0, AL=transferred	CHS; max ~8GB.
03h	Write sectors	Same as 02h	Same	Destructive.
04h	Verify sectors	Same as 02h (no buf)	Same	No data transfer.
05h	Format track	DL=drive, CH=track, DH=head, ES:BX=address field buffer	CF=0	Floppy; ES:BX points to format buffer defining sector layout.
08h	Get params	DL=drive	CH=low 8 bits of max cyl, CL bits 0-5=max sector (1-63), bits 6-7=high 2 bits of max cyl, DH=max head, DL=#drives; ES:DI=EDD params	-
0Fh	Set disk type	AL=media type	None	Floppy format prep.
15h	Get disk type	DL=drive	AH=00h (not present), 01h (floppy no change detect), 02h (floppy with change detect), 03h (hard disk); CX:DX=# 512-byte sectors (for type 03h)	-
16h	Media change?	DL=drive	AH=0 changed,1 no,FF=unsupported	Floppy only.
17h	Set media type	AL=type (as 15h)	None	-
41h	Extensions check	BX=55AAh, DL=drive	BX=AA55h (verify BX changed from 55AAh), AH=vers, CX=flags (bit0=LBA)	EDD support.
42h	Ext read (LBA)	DS:SI=DAP (Disk Addr Packet); DAP structure at DS:SI: Offset 00h: byte - packet size (10h or 18h); 01h: byte - reserved (0); 02h: word - #sectors; 04h: word - buffer offset; 06h: word - buffer segment; 08h: qword - starting LBA	CF=0	LBA48 in later BIOS.
43h	Ext write	Same as 42h	Same	-
44h	Ext verify	Same as 42h	Same	-
45h	Lock/unlock	DL=drive, AL=0 (unlock) or 1 (lock)	CF=0	Prevent eject.
46h	Eject media	DL=drive	CF=0	Removable only.
47h	Ext seek	Same as 42h (no data)	CF=0	-
48h	Get ext params	DS:SI=DAP (size=18h)	ES:DI=drive info	Size, geometry.
4Eh	Set config	DS:SI=cfg data	CF=0	Advanced.

INT 14h: Serial Port Services

RS-232 comms. Port in DX (0=COM1,1=COM2).

AH	Function	Inputs	Outputs	Notes
00h	Init port	DX=port, AL=params (bits 7-5: baud rate, 4-3: parity, 2: stop bits, 1-0: word length). Example: E3h = 9600,N,8,1	None	Bits: AL lo=word len (3=8bit).
01h	Transmit char	DX=port, AL=char	None	Waits if full.
02h	Receive char	DX=port	AL=char, if none CF=1	-
03h	Get status	DX=port	AH=modem, AL=line (bits:0=RxRDY,1=TxRDY,2=break,3=overrun,4=parity err,5=framing,6=Tx empty,7=Rx full)	Modem: bits for DTR/DSR etc.

INT 15h: System Services

Misc; subfunctions in AH (or full AX for some, e.g., A20). Includes A20 gate control for memory access.

AH	Function	Inputs	Outputs	Notes
00h	Cassette (obsolete)	-	-	Rarely used.
4Fh	Keyboard intercept	-	-	For custom keys.
76h	Sys Req	-	-	-
80h8Fh	OEM-specific	Varies	Varies	Vendor hooks.
24h (AX=2400h)	Disable A20	Set AX=2400h	CF=0 success, AH=status	For <1MB compatibility.
24h (AX=2401h)	Enable A20	Set AX=2401h	CF=0 success, AH=status	Required for >1MB access.
24h (AX=2403h)	Query A20 status	Set AX=2403h	CF=0, AX=status (0=disabled, 1=enabled)	Check before enable.
86h	Delay	CX:DX=s as 32-bit value (CX=high word, DX=low word)	None	Max ~71 minutes (32-bit s limit: FFFFFFFFh s 4295 seconds).
87h	Block move (up to 1MB)	ES:SI=GDT pointer, CX=word count	CF=0	For moving data between conventional and extended memory using a GDT structure.
88h	Get ext mem	None	AX=KB (>1MB)	Up to 64MB typical.
89h	Enter protected mode	-	-	BIOS-specific.
C0hFFh	System configuration & modern extensions	Varies	Varies	Includes PnP/PCI, pointing device, etc.
E801h	Get mem map	-	AX=extended memory 1-16MB (KB), BX=extended memory >16MB (64KB blocks), CX=configured memory 1-16MB (KB), DX=configured memory >16MB (64KB blocks)	Legacy mem query.
E820h	Memory map	EAX=0E820h, EDX='SMAP' (534D4150h), ES:DI=buf (20-byte entry), ECX=size (20), EBX=continuation (0 first call)	EAX='SMAP', EBX=continuation value (0 if last entry), ECX=bytes returned, CF=0	Iter: Call repeatedly with EBX from previous call until EBX=0. EDX must be 'SMAP' (534D4150h) on each call. CF=1 or EAX'SMAP' indicates error/unsupported. Buf has base,len,type (1=usable RAM, 2=reserved, 3=ACPI reclaimable, 4=ACPI NVS, 5=bad memory). Best for modern.

INT 16h: Keyboard Services

Waits/polls keys. Returns AL=ASCII, AH=scan.

AH	Function	Inputs	Outputs	Notes
00h	Get key (wait)	None	AL=char, AH=scan	Blocks until key.
01h	Check key	None	ZF=0 if key (AL=char, AH=scan), else ZF=1	Non-blocking.
02h	Get shift status	None	AL=flags (bit0=right shift,1=left,2=ctrl,3=alt,4=scroll,5=num,6=caps,7=insert)	-
05h	Buffer key	AL=char, AH=scan	None	Simulate press.
10h	Ext get key	None	AL=char, AH=scan (ext keys >80h)	Unicode/Ext support.
11h	Ext check	None	As 01h	-
12h	Ext shift	None	As 02h	-

INT 17h: Printer Services

Parallel port. DX=printer (0=LPT1).

AH	Function	Inputs	Outputs	Notes
00h	Print char	DX=port, AL=char	AH=status (bit0=timeout,13=err,4=selected,5=out empty,6=paper out,7=ack)	-
01h	Init port	DX=port	AH=status	Reset.
02h	Get status	DX=port	AH=status (as above)	-
03h	-	-	-	Unused.

INT 18h: User Wait / ROM BASIC

AH	Function	Inputs	Outputs	Notes
00h	User wait	None	None	Infinite loop; for BASIC entry.
-	Boot BASIC	None	-	Jumps to ROM BASIC if present (e.g., IBM PC).

INT 19h: Bootstrap Loader

AH	Function	Inputs	Outputs	Notes
-	Reboot	None	-	Warm boot; restarts POST/boot sequence.

INT 1Ah: Time-of-Day Services

RTC access (AT+).

AH	Function	Inputs	Outputs	Notes
00h	Get time	None	CX:DX = tick count (ticks since midnight at 18.2 Hz)	-
01h	Set time	CX:DX = tick count to set (ticks since midnight at 18.2 Hz)	None	-
02h	Get RTC time	None	CH=hr, CL=min, DH=sec (BCD)	24hr format.
03h	Set RTC time	As 02h	CF=0	-
04h	Get RTC date	None	CH=month, CL=day, DH=year (BCD, year=099)	-
05h	Set RTC date	As 04h	CF=0	-
06h	Set alarm	As 02h	CF=0	IRQ 4Fh trigger.
07h	Reset alarm	None	None	-

Other Notable Interrupts

• INT 05h: Print screen (Shift-PrtSc).
• INT 09h: Hardware keyboard IRQ (low-level; avoid in software).
• INT 1Bh: Ctrl-Break handler (hook for abort).
• INT 1Ch: Timer tick (18.2 Hz; hook for delays).
• INT 1Dh: Video param table (seg addr in IVT).
• INT 1Eh: Disk param table (seg addr).
• INT 1Fh: Graphics char table (seg addr).

For full details (e.g., rare subfunctions), consult RBIL. Test on emulators like QEMU for consistency. These enable hardware access without drivers in early boot.

BIOS Interrupt Compatibility Table

By System Generation

Interrupt/Function	IBM PC (1981)	IBM XT (1983)	IBM AT (1984)	PS/2 (1987)	Modern BIOS	Notes
INT 10h (Video)
AH=00h-0Fh	X	X	X	X	X	Core functions universal
AH=10h (Palette)			EGA	X	X	Requires EGA/VGA
AH=11h (Font)			EGA	X	X	Requires EGA/VGA
AH=12h (Alt func)			EGA	X	X	Requires EGA/VGA
AH=13h (Write str)			X	X	X	AT+ only
AH=4Fh (VBE)					1990s+	VESA BIOS Ext required
INT 11h (Equipment)	X	X	X	X	X	Universal
INT 12h (Memory)	X	X	X	X	X	Universal
INT 13h (Disk)
AH=00h-05h	X	X	X	X	X	Core CHS functions
AH=08h (Get params)	Partial	X	X	X	X	Limited on PC
AH=15h (Get type)			X	X	X	AT+ only
AH=41h-48h (Ext/LBA)					1994+	Phoenix EDD spec
INT 14h (Serial)	X	X	X	X	X	Universal
INT 15h (System)
AH=00h (Cassette)	X	X				PC/XT only
AH=4Fh (Kbd intercept)			X	X	X	AT+
AH=86h (Delay)			X	X	X	AT+
AH=87h (Block move)			X	X	X	AT+ (286+)
AH=88h (Ext mem)			X	X	X	AT+ (286+)
AH=89h (Prot mode)			X	X	Rare	AT+, BIOS-specific
AX=2400h-2403h (A20)				X	X	PS/2+
AH=C0h (Sys config)			X	X	X	AT+
AH=E801h (Mem map)					1990s+	Phoenix spec
AH=E820h (Mem map)					1995+	ACPI spec, best method
INT 16h (Keyboard)
AH=00h-02h	X	X	X	X	X	Universal
AH=05h (Buffer key)			X	X	X	AT+
AH=10h-12h (Extended)				X	X	PS/2+ (101-key kbd)
INT 17h (Printer)	X	X	X	X	X	Universal
INT 1Ah (RTC)
AH=00h-01h (Tick)	X	X	X	X	X	Universal
AH=02h-07h (RTC)			X	X	X	AT+ (MC146818 RTC)

Key Hardware Requirements

Feature	Minimum Hardware	Year Introduced
CHS disk access	Floppy/HDD controller	1981 (PC)
Hard disk support	XT controller	1983
Extended memory access	286+ CPU	1984 (AT)
EGA/VGA functions	EGA/VGA card	1984/1987
101-key keyboard	Enhanced keyboard	1987 (PS/2)
A20 gate control	PS/2 keyboard controller	1987
LBA disk access	Phoenix BIOS extensions	1994
VESA VBE	VESA BIOS	1989+
E820 memory map	ACPI-aware BIOS	1995+

Quick Compatibility Guide

For maximum compatibility (PC/XT/AT):

• INT 10h: AH=00h-0Fh (avoid 10h-13h, 4Fh)
• INT 13h: AH=00h-05h (CHS only)
• INT 16h: AH=00h-02h only
• INT 1Ah: AH=00h-01h (tick count, not RTC)

For AT-compatible systems (1984+):

• Add: INT 10h AH=10h-13h (EGA/VGA)
• Add: INT 13h AH=08h, 15h (reliable drive params)
• Add: INT 15h AH=86h-88h (delay, extended memory)
• Add: INT 1Ah AH=02h-07h (RTC access)

For modern bootloaders (1995+):

• Use: INT 15h E820h for memory detection
• Use: INT 13h AH=41h-48h for LBA disk access
• Use: INT 10h AH=4Fh for VESA graphics
• Use: A20 gate (AX=2401h) before accessing >1MB