How To Divide A Register By A Number Assembly

x86 Assembly Guide

Contents: Registers | Retention and Addressing | Instructions | Calling Convention

This is a version adapted by Quentin Carbonneaux from David Evans' original certificate. The syntax was changed from Intel to AT&T, the standard syntax on UNIX systems, and the HTML code was purified.

This guide describes the basics of 32-flake x86 assembly language programming, covering a small but useful subset of the available instructions and assembler directives. In that location are several unlike assembly languages for generating x86 machine code. The i nosotros will use in CS421 is the GNU Assembler (gas) assembler. We will uses the standard AT&T syntax for writing x86 assembly lawmaking.

The full x86 pedagogy set is large and complex (Intel's x86 instruction set up manuals comprise over 2900 pages), and we exercise not cover it all in this guide. For example, in that location is a 16-scrap subset of the x86 instruction gear up. Using the 16-fleck programming model can be quite complex. It has a segmented memory model, more restrictions on annals usage, and and then on. In this guide, nosotros will limit our attention to more modern aspects of x86 programming, and delve into the teaching fix only in plenty detail to get a basic feel for x86 programming.

Registers

Modern (i.eastward 386 and across) x86 processors have 8 32-bit general purpose registers, as depicted in Figure ane. The annals names are mostly historical. For example, EAX used to be chosen the accumulator since information technology was used past a number of arithmetic operations, and ECX was known equally the counter since it was used to hold a loop index. Whereas most of the registers have lost their special purposes in the mod pedagogy fix, past convention, ii are reserved for special purposes — the stack pointer (ESP) and the base pointer (EBP).

For the EAX, EBX, ECX, and EDX registers, subsections may be used. For instance, the least significant two bytes of EAX can be treated as a sixteen-bit register called AX. The least significant byte of AX can be used as a unmarried 8-bit register chosen AL, while the most significant byte of AX can be used as a single viii-bit register called AH. These names refer to the same physical annals. When a ii-byte quantity is placed into DX, the update affects the value of DH, DL, and EDX. These sub-registers are mainly hold-overs from older, xvi-bit versions of the education set up. However, they are sometimes convenient when dealing with information that are smaller than 32-bits (e.g. 1-byte ASCII characters).

Figure 1. x86 Registers

Memory and Addressing Modes

Declaring Static Data Regions

You can declare static information regions (coordinating to global variables) in x86 associates using special assembler directives for this purpose. Data declarations should be preceded by the .data directive. Following this directive, the directives .byte, .short, and .long can be used to declare 1, 2, and iv byte data locations, respectively. To refer to the address of the information created, we tin label them. Labels are very useful and versatile in associates, they give names to memory locations that volition be figured out afterwards past the assembler or the linker. This is similar to declaring variables by name, but abides by some lower level rules. For case, locations alleged in sequence will be located in memory next to one another.

Example declarations:

.data

var:

.byte 64 /* Declare a byte, referred to as location var, containing the value 64. */

.byte x /* Declare a byte with no characterization, containing the value 10. Its location is var + 1. */

x:

.brusk 42 /* Declare a 2-byte value initialized to 42, referred to as location x. */

y:

.long 30000 /* Declare a 4-byte value, referred to as location y, initialized to 30000. */

Unlike in high level languages where arrays tin can have many dimensions and are accessed by indices, arrays in x86 assembly language are simply a number of cells located contiguously in memory. An array tin be declared by just list the values, as in the kickoff example below. For the special case of an array of bytes, string literals tin can be used. In example a large area of memory is filled with zeroes the .nix directive tin can be used.

Some examples:

southward:

.long 1, 2, three /* Declare iii 4-byte values, initialized to ane, 2, and 3.
The value at location south + 8 volition exist 3. */

barr:

.zero 10 /* Declare ten bytes starting at location barr, initialized to 0. */

str:

.string "hello" /* Declare 6 bytes starting at the address str initialized to
the ASCII graphic symbol values for howdy followed by a nul (0) byte. */

Addressing Memory

Modern x86-uniform processors are capable of addressing up to 2³² bytes of memory: memory addresses are 32-bits wide. In the examples above, where we used labels to refer to memory regions, these labels are actually replaced past the assembler with 32-bit quantities that specify addresses in memory. In add-on to supporting referring to retention regions by labels (i.e. constant values), the x86 provides a flexible scheme for calculating and referring to retention addresses: up to two of the 32-bit registers and a 32-bit signed constant tin be added together to compute a memory accost. Ane of the registers tin can be optionally pre-multiplied by 2, 4, or eight.

The addressing modes can be used with many x86 instructions (we'll describe them in the adjacent section). Here we illustrate some examples using the mov educational activity that moves information betwixt registers and memory. This instruction has ii operands: the first is the source and the 2d specifies the destination.

Some examples of mov instructions using accost computations are:

mov (%ebx), %eax /* Load 4 bytes from the retentiveness address in EBX into EAX. */

mov %ebx, var(,one) /* Move the contents of EBX into the 4 bytes at retentivity accost var.
(Note, var is a 32-bit abiding). */

mov -4(%esi), %eax /* Motility 4 bytes at memory address ESI + (-iv) into EAX. */

mov %cl, (%esi,%eax,1) /* Move the contents of CL into the byte at address ESI+EAX. */

mov (%esi,%ebx,4), %edx /* Motility the 4 bytes of data at accost ESI+iv*EBX into EDX. */

Some examples of invalid address calculations include:

mov (%ebx,%ecx,-1), %eax /* Can just add annals values. */

mov %ebx, (%eax,%esi,%edi,ane) /* At most two registers in accost computation. */

Performance Suffixes

In general, the intended size of the of the data item at a given memory address can be inferred from the assembly code didactics in which it is referenced. For instance, in all of the to a higher place instructions, the size of the memory regions could exist inferred from the size of the register operand. When we were loading a 32-fleck register, the assembler could infer that the region of retention we were referring to was 4 bytes broad. When we were storing the value of a one byte register to memory, the assembler could infer that we wanted the accost to refer to a single byte in memory.

Notwithstanding, in some cases the size of a referred-to memory region is ambiguous. Consider the pedagogy mov $2, (%ebx). Should this instruction move the value 2 into the single byte at address EBX? Perhaps it should move the 32-bit integer representation of 2 into the iv-bytes starting at address EBX. Since either is a valid possible interpretation, the assembler must exist explicitly directed as to which is correct. The size prefixes b, w, and l serve this purpose, indicating sizes of 1, 2, and 4 bytes respectively.

For case:

movb $2, (%ebx) /* Move 2 into the single byte at the address stored in EBX. */

movw $2, (%ebx) /* Motion the xvi-bit integer representation of 2 into the 2 bytes starting at the address in EBX. */

movl $2, (%ebx) /* Movement the 32-bit integer representation of ii into the 4 bytes starting at the address in EBX. */

Instructions

Motorcar instructions generally fall into three categories: data movement, arithmetics/logic, and command-flow. In this section, we will look at important examples of x86 instructions from each category. This section should not be considered an exhaustive list of x86 instructions, but rather a useful subset. For a complete list, see Intel's educational activity set reference.

We use the following annotation:

<reg32> Any 32-bit register (%eax, %ebx, %ecx, %edx, %esi, %edi, %esp, or %ebp)

<reg16> Any xvi-bit register (%ax, %bx, %cx, or %dx)

<reg8> Any viii-flake register (%ah, %bh, %ch, %dh, %al, %bl, %cl, or %dl)

<reg> Any register

<mem> A retentivity accost (eastward.chiliad., (%eax), 4+var(,i), or (%eax,%ebx,1))

<con32> Any 32-bit immediate

<con16> Whatever 16-flake immediate

<con8> Whatever 8-bit immediate

<con> Any 8-, sixteen-, or 32-scrap immediate

In assembly language, all the labels and numeric constants used as immediate operands (i.e. not in an address calculation like 3(%eax,%ebx,eight)) are always prefixed by a dollar sign. When needed, hexadecimal notation can exist used with the 0x prefix (e.k. $0xABC). Without the prefix, numbers are interpreted in the decimal footing.

Information Motion Instructions

mov — Move

The mov instruction copies the data detail referred to by its kickoff operand (i.e. register contents, retentivity contents, or a constant value) into the location referred to by its 2d operand (i.due east. a register or retentivity). While register-to-register moves are possible, directly memory-to-memory moves are not. In cases where memory transfers are desired, the source memory contents must outset be loaded into a register, then tin exist stored to the destination memory address.
Syntax
mov <reg>, <reg>
mov <reg>, <mem>
mov <mem>, <reg>
mov <con>, <reg>
mov <con>, <mem>

Examples
mov %ebx, %eax — re-create the value in EBX into EAX
movb $5, var(,1) — store the value 5 into the byte at location var

push — Push on stack

The push instruction places its operand onto the pinnacle of the hardware supported stack in memory. Specifically, push first decrements ESP by iv, and then places its operand into the contents of the 32-bit location at address (%esp). ESP (the stack pointer) is decremented past push since the x86 stack grows down — i.e. the stack grows from loftier addresses to lower addresses.
Syntax
push <reg32>
button <mem>
push <con32>

Examples
push %eax — push eax on the stack
push var(,one) — push the 4 bytes at address var onto the stack

pop — Pop from stack

The pop instruction removes the 4-byte data element from the superlative of the hardware-supported stack into the specified operand (i.due east. annals or retentiveness location). Information technology first moves the 4 bytes located at memory location (%esp) into the specified register or retentiveness location, and and so increments ESP by 4.
Syntax
pop <reg32>
popular <mem>
Examples
pop %edi — popular the meridian element of the stack into EDI.
pop (%ebx) — popular the top element of the stack into memory at the four bytes starting at location EBX.

lea — Load effective address

The lea instruction places the address specified by its first operand into the register specified by its second operand. Note, the contents of the memory location are not loaded, only the effective accost is computed and placed into the annals. This is useful for obtaining a pointer into a memory region or to perform uncomplicated arithmetic operations.
Syntax
lea <mem>, <reg32>

Examples
lea (%ebx,%esi,8), %edi — the quantity EBX+8*ESI is placed in EDI.
lea val(,1), %eax — the value val is placed in EAX.

Arithmetic and Logic Instructions

add together — Integer improver

The add instruction adds together its two operands, storing the effect in its second operand. Note, whereas both operands may be registers, at most one operand may exist a memory location.
Syntax
add together <reg>, <reg>
add together <mem>, <reg>
add <reg>, <mem>
add <con>, <reg>
add together <con>, <mem>

Examples
add $10, %eax — EAX is ready to EAX + 10
addb $10, (%eax) — add together 10 to the single byte stored at retentiveness address stored in EAX

sub — Integer subtraction

The sub instruction stores in the value of its second operand the result of subtracting the value of its first operand from the value of its 2d operand. As with add, whereas both operands may exist registers, at most ane operand may be a retention location.
Syntax
sub <reg>, <reg>
sub <mem>, <reg>
sub <reg>, <mem>
sub <con>, <reg>
sub <con>, <mem>

Examples
sub %ah, %al — AL is set to AL - AH
sub $216, %eax — subtract 216 from the value stored in EAX

inc, dec — Increase, Decrement

The inc instruction increments the contents of its operand by one. The dec instruction decrements the contents of its operand by one.
Syntax
inc <reg>
inc <mem>
dec <reg>
dec <mem>

Examples
dec %eax — decrease one from the contents of EAX
incl var(,i) — add together ane to the 32-bit integer stored at location var

imul — Integer multiplication

The imul pedagogy has two basic formats: two-operand (starting time two syntax listings higher up) and 3-operand (last two syntax listings above).
The two-operand course multiplies its two operands together and stores the result in the second operand. The result (i.eastward. second) operand must be a annals.

The 3 operand form multiplies its 2d and 3rd operands together and stores the outcome in its terminal operand. Once again, the result operand must be a register. Furthermore, the first operand is restricted to existence a constant value.

Syntax
imul <reg32>, <reg32>
imul <mem>, <reg32>
imul <con>, <reg32>, <reg32>
imul <con>, <mem>, <reg32>

Examples

imul (%ebx), %eax — multiply the contents of EAX past the 32-bit contents of the memory at location EBX. Store the result in EAX.

imul $25, %edi, %esi — ESI is prepare to EDI * 25

idiv — Integer division

The idiv instruction divides the contents of the 64 bit integer EDX:EAX (constructed by viewing EDX as the most meaning four bytes and EAX as the to the lowest degree meaning four bytes) by the specified operand value. The quotient result of the sectionalization is stored into EAX, while the remainder is placed in EDX.
Syntax
idiv <reg32>
idiv <mem>

Examples

idiv %ebx — divide the contents of EDX:EAX past the contents of EBX. Place the quotient in EAX and the residue in EDX.

idivw (%ebx) — divide the contents of EDX:EAS past the 32-bit value stored at the retention location in EBX. Place the quotient in EAX and the remainder in EDX.

and, or, xor — Bitwise logical and, or, and exclusive or

These instructions perform the specified logical operation (logical bitwise and, or, and exclusive or, respectively) on their operands, placing the result in the first operand location.
Syntax
and <reg>, <reg>
and <mem>, <reg>
and <reg>, <mem>
and <con>, <reg>
and <con>, <mem>

or <reg>, <reg>
or <mem>, <reg>
or <reg>, <mem>
or <con>, <reg>
or <con>, <mem>

xor <reg>, <reg>
xor <mem>, <reg>
xor <reg>, <mem>
xor <con>, <reg>
xor <con>, <mem>

Examples
and $0x0f, %eax — articulate all merely the concluding 4 bits of EAX.
xor %edx, %edx — set the contents of EDX to zero.

non — Bitwise logical non

Logically negates the operand contents (that is, flips all bit values in the operand).
Syntax
not <reg>
not <mem>

Example
not %eax — flip all the bits of EAX

neg — Negate

Performs the two's complement negation of the operand contents.
Syntax
neg <reg>
neg <mem>

Example
neg %eax — EAX is set to (- EAX)

shl, shr — Shift left and correct

These instructions shift the $.25 in their offset operand's contents left and right, padding the resulting empty bit positions with zeros. The shifted operand can be shifted up to 31 places. The number of bits to shift is specified by the second operand, which can be either an 8-bit constant or the register CL. In either case, shifts counts of greater then 31 are performed modulo 32.
Syntax
shl <con8>, <reg>
shl <con8>, <mem>
shl %cl, <reg>
shl %cl, <mem>

shr <con8>, <reg>
shr <con8>, <mem>
shr %cl, <reg>
shr %cl, <mem>

Examples

shl $1, eax — Multiply the value of EAX by ii (if the well-nigh significant bit is 0)

shr %cl, %ebx — Store in EBX the flooring of effect of dividing the value of EBX by 2ⁿ where n is the value in CL. Caution: for negative integers, information technology is unlike from the C semantics of division!

Control Catamenia Instructions

The x86 processor maintains an instruction pointer (EIP) annals that is a 32-bit value indicating the location in memory where the current instruction starts. Normally, it increments to point to the side by side education in memory begins after execution an didactics. The EIP annals cannot be manipulated directly, merely is updated implicitly by provided control flow instructions.

We employ the notation <characterization> to refer to labeled locations in the program text. Labels can be inserted anywhere in x86 assembly code text by inbound a characterization name followed by a colon. For case,

            mov viii(%ebp), %esi begin:        xor %ecx, %ecx        mov (%esi), %eax

The second instruction in this code fragment is labeled begin. Elsewhere in the code, we tin can refer to the retentivity location that this pedagogy is located at in memory using the more convenient symbolic proper noun begin. This label is merely a convenient way of expressing the location instead of its 32-chip value.

jmp — Jump

Transfers program control catamenia to the teaching at the memory location indicated by the operand.
Syntax
jmp <characterization>

Instance
jmp begin — Jump to the instruction labeled begin.

jcondition — Conditional bound

These instructions are conditional jumps that are based on the status of a set of status codes that are stored in a special register called the car status word. The contents of the auto status word include information most the concluding arithmetic operation performed. For case, ane flake of this word indicates if the concluding effect was zero. Another indicates if the last result was negative. Based on these condition codes, a number of conditional jumps can be performed. For example, the jz didactics performs a jump to the specified operand label if the result of the concluding arithmetic operation was zero. Otherwise, control gain to the adjacent instruction in sequence.
A number of the conditional branches are given names that are intuitively based on the concluding operation performed being a special compare pedagogy, cmp (see beneath). For instance, provisional branches such as jle and jne are based on first performing a cmp operation on the desired operands.

Syntax
je <label> (bound when equal)
jne <label> (jump when not equal)
jz <characterization> (jump when last consequence was zero)
jg <label> (bound when greater than)
jge <label> (jump when greater than or equal to)
jl <label> (spring when less than)
jle <label> (jump when less than or equal to)

Example
cmp %ebx, %eax jle done          
If the contents of EAX are less than or equal to the contents of EBX, jump to the characterization done. Otherwise, keep to the side by side instruction.

cmp — Compare

Compare the values of the 2 specified operands, setting the condition codes in the automobile status word accordingly. This educational activity is equivalent to the sub teaching, except the result of the subtraction is discarded instead of replacing the first operand.
Syntax
cmp <reg>, <reg>
cmp <mem>, <reg>
cmp <reg>, <mem>
cmp <con>, <reg>

Example
cmpb $10, (%ebx)

jeq loop

If the byte stored at the memory location in EBX is equal to the integer abiding 10, leap to the location labeled loop.

call, ret — Subroutine telephone call and return

These instructions implement a subroutine call and render. The call didactics kickoff pushes the electric current code location onto the hardware supported stack in memory (come across the push button instruction for details), and then performs an unconditional spring to the code location indicated by the label operand. Different the simple jump instructions, the phone call instruction saves the location to render to when the subroutine completes.
The ret educational activity implements a subroutine return machinery. This teaching first pops a lawmaking location off the hardware supported in-memory stack (run into the popular educational activity for details). It then performs an unconditional jump to the retrieved code location.

Syntax
call <characterization>
ret

Calling Convention

To allow separate programmers to share code and develop libraries for use by many programs, and to simplify the utilise of subroutines in general, programmers typically prefer a common calling convention. The calling convention is a protocol about how to call and return from routines. For example, given a ready of calling convention rules, a developer demand not examine the definition of a subroutine to determine how parameters should be passed to that subroutine. Furthermore, given a set of calling convention rules, high-level linguistic communication compilers can be made to follow the rules, thus allowing hand-coded assembly language routines and high-level linguistic communication routines to call one another.

In exercise, many calling conventions are possible. We will describe the widely used C language calling convention. Following this convention will allow you to write assembly language subroutines that are safely callable from C (and C++) code, and will also enable you lot to call C library functions from your assembly linguistic communication lawmaking.

The C calling convention is based heavily on the use of the hardware-supported stack. Information technology is based on the push, pop, phone call, and ret instructions. Subroutine parameters are passed on the stack. Registers are saved on the stack, and local variables used past subroutines are placed in memory on the stack. The vast majority of high-level procedural languages implemented on nearly processors have used like calling conventions.

The calling convention is cleaved into two sets of rules. The first set up of rules is employed by the caller of the subroutine, and the second set of rules is observed by the writer of the subroutine (the callee). It should exist emphasized that mistakes in the observance of these rules quickly result in fatal program errors since the stack will exist left in an inconsistent state; thus meticulous care should be used when implementing the call convention in your ain subroutines.

Stack during Subroutine Call

[Thank you to James Peterson for finding and fixing the bug in the original version of this figure!]

A skillful way to visualize the operation of the calling convention is to draw the contents of the nearby region of the stack during subroutine execution. The image above depicts the contents of the stack during the execution of a subroutine with 3 parameters and 3 local variables. The cells depicted in the stack are 32-chip broad memory locations, thus the memory addresses of the cells are 4 bytes autonomously. The beginning parameter resides at an offset of 8 bytes from the base of operations pointer. Above the parameters on the stack (and beneath the base pointer), the call instruction placed the render address, thus leading to an extra iv bytes of offset from the base pointer to the first parameter. When the ret pedagogy is used to render from the subroutine, it will jump to the render address stored on the stack.

Caller Rules

To make a subrouting phone call, the caller should:

Before calling a subroutine, the caller should save the contents of certain registers that are designated caller-saved. The caller-saved registers are EAX, ECX, EDX. Since the called subroutine is allowed to modify these registers, if the caller relies on their values later the subroutine returns, the caller must push the values in these registers onto the stack (then they can be restore after the subroutine returns.
To laissez passer parameters to the subroutine, push them onto the stack earlier the call. The parameters should be pushed in inverted social club (i.east. last parameter get-go). Since the stack grows down, the first parameter volition be stored at the everyman address (this inversion of parameters was historically used to allow functions to be passed a variable number of parameters).
To call the subroutine, use the call instruction. This didactics places the render address on meridian of the parameters on the stack, and branches to the subroutine code. This invokes the subroutine, which should follow the callee rules below.

After the subroutine returns (immediately post-obit the phone call pedagogy), the caller can expect to find the return value of the subroutine in the register EAX. To restore the machine land, the caller should:

Remove the parameters from stack. This restores the stack to its state before the call was performed.
Restore the contents of caller-saved registers (EAX, ECX, EDX) by popping them off of the stack. The caller tin assume that no other registers were modified past the subroutine.

Case

The code beneath shows a part call that follows the caller rules. The caller is calling a function myFunc that takes three integer parameters. First parameter is in EAX, the 2nd parameter is the constant 216; the third parameter is in the retentivity location stored in EBX.

push (%ebx)    /* Push final parameter beginning */ push button $216      /* Push the second parameter */ push %eax      /* Push starting time parameter last */  call myFunc    /* Call the role (presume C naming) */  add together $12, %esp

Note that afterwards the telephone call returns, the caller cleans upwards the stack using the add pedagogy. We accept 12 bytes (3 parameters * iv bytes each) on the stack, and the stack grows downward. Thus, to get rid of the parameters, we tin can simply add together 12 to the stack pointer.

The issue produced past myFunc is now available for use in the register EAX. The values of the caller-saved registers (ECX and EDX), may have been changed. If the caller uses them after the telephone call, information technology would have needed to save them on the stack before the call and restore them after it.

Callee Rules

The definition of the subroutine should adhere to the following rules at the beginning of the subroutine:

Push button the value of EBP onto the stack, and and then copy the value of ESP into EBP using the following instructions:
```
              push %ebp     mov  %esp, %ebp            
```
This initial activity maintains the base of operations pointer, EBP. The base pointer is used past convention as a point of reference for finding parameters and local variables on the stack. When a subroutine is executing, the base pointer holds a re-create of the stack pointer value from when the subroutine started executing. Parameters and local variables will always be located at known, constant offsets away from the base pointer value. We push the old base arrow value at the first of the subroutine so that we tin later restore the appropriate base pointer value for the caller when the subroutine returns. Remember, the caller is not expecting the subroutine to change the value of the base pointer. We then move the stack pointer into EBP to obtain our point of reference for accessing parameters and local variables.
Next, classify local variables past making space on the stack. Recall, the stack grows down, then to make space on the acme of the stack, the stack pointer should exist decremented. The amount past which the stack arrow is decremented depends on the number and size of local variables needed. For case, if 3 local integers (4 bytes each) were required, the stack pointer would demand to be decremented by 12 to make space for these local variables (i.e., sub $12, %esp). As with parameters, local variables will be located at known offsets from the base arrow.
Next, save the values of the callee-saved registers that volition be used by the function. To salve registers, button them onto the stack. The callee-saved registers are EBX, EDI, and ESI (ESP and EBP will too be preserved by the calling convention, only demand not be pushed on the stack during this step).

After these three actions are performed, the body of the subroutine may continue. When the subroutine is returns, it must follow these steps:

Leave the return value in EAX.
Restore the old values of any callee-saved registers (EDI and ESI) that were modified. The annals contents are restored by popping them from the stack. The registers should be popped in the inverse lodge that they were pushed.
Deallocate local variables. The obvious way to do this might be to add the appropriate value to the stack pointer (since the infinite was allocated by subtracting the needed amount from the stack pointer). In practice, a less error-prone way to deallocate the variables is to move the value in the base pointer into the stack pointer: mov %ebp, %esp. This works considering the base pointer always contains the value that the stack pointer contained immediately prior to the allocation of the local variables.
Immediately earlier returning, restore the caller's base pointer value by popping EBP off the stack. Recall that the first matter we did on entry to the subroutine was to push the base of operations arrow to salvage its quondam value.
Finally, render to the caller by executing a ret didactics. This teaching volition find and remove the appropriate render accost from the stack.

Note that the callee's rules fall cleanly into 2 halves that are basically mirror images of one another. The first one-half of the rules use to the beginning of the role, and are unremarkably said to define the prologue to the function. The latter half of the rules apply to the terminate of the function, and are thus commonly said to ascertain the epilogue of the part.

Example

Hither is an case function definition that follows the callee rules:

            /* Kickoff the code department */   .text    /* Define myFunc as a global (exported) function. */   .globl myFunc   .blazon myFunc, @function myFunc:    /* Subroutine Prologue */   push %ebp      /* Save the old base pointer value. */   mov %esp, %ebp /* Set the new base pointer value. */   sub $4, %esp   /* Make room for one iv-byte local variable. */   button %edi      /* Save the values of registers that the function */   push %esi      /* volition alter. This function uses EDI and ESI. */   /* (no need to salvage EBX, EBP, or ESP) */    /* Subroutine Body */   mov eight(%ebp), %eax   /* Move value of parameter 1 into EAX. */   mov 12(%ebp), %esi  /* Move value of parameter 2 into ESI. */   mov 16(%ebp), %edi  /* Move value of parameter 3 into EDI. */    mov %edi, -iv(%ebp)  /* Move EDI into the local variable. */   add together %esi, -4(%ebp)  /* Add together ESI into the local variable. */   add -4(%ebp), %eax  /* Add the contents of the local variable */                       /* into EAX (final result). */    /* Subroutine Epilogue */   popular %esi       /* Recover register values. */   pop %edi   mov %ebp, %esp /* Deallocate the local variable. */   pop %ebp       /* Restore the caller's base pointer value. */   ret

The subroutine prologue performs the standard deportment of saving a snapshot of the stack arrow in EBP (the base pointer), allocating local variables by decrementing the stack pointer, and saving register values on the stack.

In the body of the subroutine we tin encounter the apply of the base pointer. Both parameters and local variables are located at constant offsets from the base pointer for the duration of the subroutines execution. In particular, we discover that since parameters were placed onto the stack earlier the subroutine was called, they are always located below the base of operations pointer (i.e. at college addresses) on the stack. The get-go parameter to the subroutine can always be found at retentiveness location (EBP+viii), the second at (EBP+12), the third at (EBP+16). Similarly, since local variables are allocated after the base pointer is fix, they always reside above the base of operations pointer (i.e. at lower addresses) on the stack. In particular, the start local variable is always located at (EBP-4), the second at (EBP-eight), and then on. This conventional employ of the base pointer allows us to quickly place the utilize of local variables and parameters within a function body.

The role epilogue is basically a mirror image of the office prologue. The caller'south annals values are recovered from the stack, the local variables are deallocated by resetting the stack pointer, the caller's base arrow value is recovered, and the ret instruction is used to return to the appropriate code location in the caller.

Credits: This guide was originally created past Adam Ferrari many years agone,
and since updated by Alan Batson, Mike Lack, and Anita Jones.
It was revised for 216 Jump 2006 by David Evans.
It was finally modified by Quentin Carbonneaux to use the AT&T syntax for Yale's CS421.