x86 assembly programming in real mode is a type of assembly computer programming for the Intel x86 in real mode. It involves the manipulation of several 16-bit processor registers, and dealing with physical addresses in memory only (as opposed to protected mode). Perhaps the most popular use of this type of programming was writing DOS programs in the 1980s. All modern x86 operating systems use protected mode; however, when the computer boots, it starts up in real mode, so the part of the operating system responsible for switching into protected mode must operate in the real mode environment. Assembly language or simply assembly is a human-readable notation for the machine language that a specific computer architecture uses. ... Jump to: navigation, search Wikibooks has more about this subject: Computer programming Computer programming (often simply programming) is the craft of implementing one or more interrelated abstract algorithms using a particular programming language to produce a concrete computer program. ... Intel Corporation (NASDAQ: INTC) (founded 1968) is a U.S.-based multinational corporation that is best known for designing and manufacturing microprocessors and specialized integrated circuits. ... x86 or 80x86 is the generic name of a microprocessor architecture first developed and manufactured by Intel. ... Real mode is an operating mode of 80286 and later x86-compatible CPUs. ... In computer architecture, a processor register is a small amount of very fast computer memory used to speed the execution of computer programs by providing quick access to commonly used values—typically, the values being in the midst of a calculation at a given point in time. ... The memory hardware (normally RAM) installed in a computer. ... Protected mode (sometimes abbreviated pmode) is an operational mode of x86-compatible CPUs of the 80286 series or later. ... Jump to: navigation, search The acronym DOS stands for disk operating system, an operating system component for computers that provides the abstraction of a file system resident on hard disk or floppy disk secondary storage. ... Jump to: navigation, search In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ... A BOOTING is any exercise of the verb to boot. ...

Registers

Each register is specialized for a certain task, and operations that deal with that task are often run more efficiently if the right register is used.

Registers in real mode include: Real mode is an operating mode of 80286 and later x86-compatible CPUs. ...

• data registers
  • AX, the accumulator
  • BX, the base register
  • CX, the counter register
  • DX, the data register

• address registers
  • SI, the source register
  • DI, the destination register
  • SP, the stack pointer register
  • BP, the stack base pointer register

Each data register can be broken up into two eight-bit registers - that is 16 bits of data in a 16 bit register can be addressed 8 bits at a time: the upper eight and the lower eight bits, and can be treated as registers in their own right. For example, in the AX register, the AH register addresses the upper eight bits of the AX register, and the AL register addresses the lower eight bits of the AX register. The other data registers can be addressed in this way by changing the suffix - "X" for extended, "H" for high, and "L" for low.

Collectively the data and address registers are called the general registers.

With the general registers, there are additionally the:

• segment registers
  • CS, the code segment register
  • DS, the data segment register
  • ES, an extra segment register
  • FS, another extra segment register (not implemented before the 80386)
  • GS, yet another extra segment register (not implemented before the 80386)
  • SS, the stack segment register

• other registers
  • IP, the instruction pointer register
  • FLAGS, the flag register

The IP register points to where in the program the processor is currently executing its code. The IP register cannot be accessed by the programmer directly. The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ... The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ...

The FLAGS register contains the current state of the processor. Each bit in this register is called a flag. Each flag can be either 1 or 0, set or not set. Some of the flags that the FLAGS register contains is carry, overflow, zero and single step. The term carry may refer to: A violation whilst dribbling in the game of basketball. ... The term arithmetic overflow or simply overflow has the following meanings. ...

Flags are notably used in the x86 architecture for comparisons. A comparison is made between two registers, for example, and in comparison of their difference a flag is raised. A jump instruction then checks the respective flag and jumps if the flag has been raised: for example x86 or 80x86 is the generic name of a microprocessor architecture first developed and manufactured by Intel. ...

 cmp ax, bx jne do_something 

first compares the AX and BX registers, and if they are unequal, the code branches off to the do_something label.

Mnemonics for opcodes

In real mode, the following mnemonics are available: aaa, aad, aam, aas, adc, add, and, call, cbw, clc, cld, cli, cmc, cmp, cmpsb, cmpsw, cwd, daa, das, dec, div, esc, hlt, idiv, imul, in, inc, int, into, iret, ja, jae, jb, jbe, jc, jcxz, je, jg, jge, jl, jle, jmp, jna, jnae, jnb, jnbe, jnc, jne, jng, jnge, jnl, jnle, jno, jnp, jns, jnz, jo, jp, jpe, jpo, js, jz, lahf, lds, lea, les, lock, lodsb, lodsw, loop, loope, loopne, loopnz, loopz, mov, movsb, movsw, mul, neg, nop, not, or, out, pop, popf, push, push, puchf, rcl, rcr, rep, repe, repne, repnz, repz, ret, rol, ror, sahf, sal, sar, sbb, scasb, scasw, shl, shr, stc, std, sti, stosb, stosw, sub, test, wait, xchg, xlat, xor INT is an assembly language instruction for x86 processors for generating a software interrupt. ... check: do long jumps and near have 4 and 2 byte offset? JMP is a mnemonic used in x86 assembly language. ... In the Intel syntax of the X86 assembly language, the MOV instruction moves data from Y to X when coded as MOV X, Y In AT&T assembler syntax, the above operation would be accomplished like so: MOVB $Y, X Either X or Y can include addressing information. ... Exclusive disjunction (usual symbol xor) is a logical operator that results in true if one of the operands (not both) is true. ...

There are also some undocumented opcodes that has no mnemonics named after them. For example, 0x0F while executed by most 8086-processors could be translated to "POP CS". Other processors in the x86-family may not interpret undocumented opcodes as earlier processors do. Therefore, use of undocumented opcodes might render your program useless in future x86-processors. Microprocessors perform operations using binary bits (on/off/1or0). ... The 8086 is a 16-bit microprocessor chip designed by Intel in 1978, which gave rise to the x86 architecture. ...

The real mode addressing model

This is quite simple, but is quite controversial amongst programmers. The x86 architecture uses a process known as segmentation to address memory, and not a linear method as used in other architectures. Segmentation involves decomposing a linear address into two parts - a segment and an offset. The segment address points to the beginning of a 64K group of addresses and an offset from the base address of the specified segment. To translate back into a linear address, the segment address is shifted 4 bits left and then added to the offset. The formula looks like this: segment*0x10+offset.

In real mode, two registers are used for a memory address: one to hold the segment, and one to hold the offset.

For example, if DS contains the hexadecimal number 0xDEAD and DX contains the number 0xCAFE they would together point to the memory address 0xDEAD * 0x10 + 0xCAFE = 0xEB5CE One quick way to do this without a hexadecimal calculator would be to just add a zero to the hexadecimal number in the segment register and then add the content of the offset register to that number. The above would be 0xDEAD0+0xCAFE. In mathematics and computer science, hexadecimal, or simply hex, is a numeral system with a radix or base of 16 usually written using the symbols 0–9 and A–F or a–f. ...

In referring to an address with a segment and an offset, the notation of segment:offset is used, in the above example, the linear address 0xEB5CE can be written as 0xDEAD:0xCAFE, or if one has a segment and offset register pair, DS:DX.

There are some special combinations of segment registers and general registers that point to important addresses:

• CS:IP points to the address where the processor will fetch its next byte of code.
• SS:SP points to the location of the last item pushed onto the stack.
• DS:SI is often used to point to data that is about to be copied to ES:DI

The PC memory layout in real mode

 0-3FF IVT (Interrupt Vector Table) 400-5FF BDA (BIOS Data Area) 600-9FFFF Ordinary application RAM A0000-BFFFF Video memory C0000-EFFFF Optional ROMs (The VGA ROM is usually located at C0000) F0000-FFFFF BIOS ROM 

Note that the BDA, Video memory, and ROMs are system architecture specific features of the original IBM PC architecture (and is retained even in modern PC compatible systems), and are not dictated by the x86 architecture itself. However, this mean that we have about 640kB of general application RAM available in real mode. Jump to: navigation, search This article is about the software. ... Jump to: navigation, search Random access memory (sometimes random-access memory), commonly known by its acronym RAM, is a type of computer storage (in practice only computer chips) whose contents can be accessed in any (i. ...

Everything above 0xFFFFF is called the "high memory area".

Interrupts in real mode

The x86 architecture is an interrupt-driven architecture. This means that hardware or software can present the processor with requested data, instead of the processor waiting for a device to respond. In computer science, an interrupt is a signal from a device which typically results in a context switch: that is, the processor sets aside what its doing and does something else. ...

There are two kinds of interrupts: software and hardware interrupts. Software interrupts are often used to talk with the operating system. A typical software interrupt is interrupt 0x21 (the DOS-interrupt, nearly all DOS system functions are accessed via this interrupt) and int3 (breakpoint, which is often used to enter a software-debugger). A typical hardware interrupt would be when some external circuit decides that it need attention from the CPU, like when the system clock ticks. The 8259 chip is used to map different IRQs into ordinary interrupts. There are two 8259 chips in a PC, 8259A and 8259B. If the 8259A chip is mapped into interrupt 0x20 to 0x27, the every time the system clock ticks the interrupt 0x20 would go off. Jump to: navigation, search In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ... 8259A is the original interrupt controller used in the IBM PC, the very first PC, it is a PIC, or programmable interrupt controller. ... An IRQ or interrupt request is the means by which hardware components request computing time from the CPU. IRQs for IBM PC compatible computers The modern IBM PC compatible computer has 16 IRQ assignments (0-15), each one representing a different physical (or virtual) piece of hardware. ...

At the very beginning of the memory lies the Interrupt Vector Table (IVT). The IVT contains pointers to all the Interrupt Service Routines (ISR's).

The pointers to the different ISR's wired to the interrupts are saved in this format:

 [offset_0][segment_0][offset_1][segment_1][... ...][offset_255][segment_255] (each integer (that is: the offset or segment-pointers) is 16 bits wide) 

There are 256 different interrupts, each with its own pointer.

Example

This NASM-assembler program is an example of real mode code that prints "Hello world!" to the screen by means of writing directly to video. Jump to: navigation, search This article is about an assembler. ...

 [org 0x100] [bits 16] [section .text] mov ax, cs ; cs = code segment mov ds, ax ; ds = cs ; (this way, we dont have to care much about where our data is located) mov ax, 0xB800 ; 0xB8000 is the base of the text video memory mov es, ax ; Remember the memory model! mov si, text ; Remember that ds:si -> es:di xor di, di ; a xor a is always zero. (di is given the value 0) around: mov al, [ds:si] ; give al the value of what ds:si points to cmp al, 0 ; compare if al contains zero ("Hello world!",0) je stop ; if so, stop writing to the screen mov [es:di], al ; move the content of al to es:di (text video memory) inc si ; select the next byte in the Hello world!-string add di, 2 ; and goto the next position on the screen. jmp around ; and go back to the beginning of the loop stop: ret ; and return back to the caller function text db "Hello world!",0 

This program could be compiled into a DOS-compatible .com-file, it is also quite possible to assemble it to any other operating system running in realmode, or even no operating system at all, but you might need to make some minor changes in such cases. Because it does not make use of the screen-functions that is provided by DOS or the BIOS, the text that the program prints to screen will disappear when the program is terminated and other programs write to video memory. Jump to: navigation, search The acronym DOS stands for disk operating system, an operating system component for computers that provides the abstraction of a file system resident on hard disk or floppy disk secondary storage. ... Jump to: navigation, search The acronym DOS stands for disk operating system, an operating system component for computers that provides the abstraction of a file system resident on hard disk or floppy disk secondary storage. ... Jump to: navigation, search This article is about the software. ...

See also

• x86 assembly language
• self-modifying code
• assembly language
• CPU design
• protected mode
• x86-assembly language in protected mode
• long mode
• x86-assembly language in longmode
• DOS