How To Move All Elements Of An Array To A Register Assembly

x86 Assembly Guide

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

This is a version adjusted 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-scrap x86 assembly language programming, covering a modest simply useful subset of the bachelor instructions and assembler directives. There are several dissimilar assembly languages for generating x86 machine code. The ane 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 instruction set is large and complex (Intel's x86 instruction fix manuals comprise over 2900 pages), and we exercise not cover it all in this guide. For example, there is a 16-bit subset of the x86 educational activity set. Using the 16-flake programming model tin be quite circuitous. Information technology has a segmented memory model, more restrictions on register usage, and and then on. In this guide, we volition limit our attention to more modern aspects of x86 programming, and delve into the pedagogy gear up simply in enough detail to become a bones feel for x86 programming.

Registers

Modern (i.due east 386 and beyond) x86 processors accept eight 32-bit general purpose registers, as depicted in Effigy 1. The register names are mostly historical. For case, EAX used to be called the accumulator since information technology was used past a number of arithmetics operations, and ECX was known as the counter since it was used to hold a loop index. Whereas most of the registers have lost their special purposes in the mod instruction set, by convention, two are reserved for special purposes — the stack pointer (ESP) and the base of operations pointer (EBP).

For the EAX, EBX, ECX, and EDX registers, subsections may be used. For example, the to the lowest degree significant 2 bytes of EAX tin can be treated as a 16-chip register chosen AX. The least significant byte of AX can be used as a single 8-fleck annals called AL, while the most significant byte of AX can be used as a single eight-scrap register called AH. These names refer to the aforementioned physical register. 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, 16-bit versions of the instruction set. Even so, they are sometimes user-friendly when dealing with data that are smaller than 32-$.25 (east.g. ane-byte ASCII characters).

Effigy 1. x86 Registers

Memory and Addressing Modes

Declaring Static Data Regions

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

Instance declarations:

.information
var:
	.byte 64	/* Declare a byte, referred to as location var, containing the value 64. */
	.byte 10	/* Declare a byte with no label, containing the value 10. Its location is var + 1. */
x:
	.short 42	/* Declare a ii-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 can have many dimensions and are accessed by indices, arrays in x86 assembly linguistic communication are but a number of cells located contiguously in memory. An array can be declared past simply listing the values, as in the first example beneath. For the special example of an array of bytes, string literals can be used. In case a large area of retention is filled with zeroes the .naught directive can be used.

Some examples:

southward:
	.long 1, 2, 3	/* Declare three 4-byte values, initialized to 1, 2, and iii. The value at location due south + 8 will exist 3. */
barr:
	.zero ten	/* Declare 10 bytes starting at location barr, initialized to 0. */
str:
	.cord "hello"	/* Declare 6 bytes starting at the address str initialized to the ASCII character values for hello followed by a nul (0) byte. */

Addressing Retention

Modern x86-compatible processors are capable of addressing up to two³² bytes of retention: retentivity addresses are 32-bits wide. In the examples in a higher place, where we used labels to refer to retentiveness regions, these labels are really replaced past the assembler with 32-flake quantities that specify addresses in memory. In addition to supporting referring to memory regions by labels (i.e. constant values), the x86 provides a flexible scheme for computing and referring to retention addresses: up to two of the 32-bit registers and a 32-bit signed abiding can exist added together to compute a memory address. Ane of the registers can exist optionally pre-multiplied by two, 4, or 8.

The addressing modes tin be used with many x86 instructions (we'll describe them in the adjacent section). Hither we illustrate some examples using the mov teaching that moves information between registers and retentivity. This teaching has ii operands: the first is the source and the second specifies the destination.

Some examples of mov instructions using accost computations are:

mov (%ebx), %eax	/* Load 4 bytes from the retention address in EBX into EAX. */
mov %ebx, var(,1)	/* Move the contents of EBX into the 4 bytes at retentivity address var. (Annotation, var is a 32-bit constant). */
mov -4(%esi), %eax	/* Move four 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,iv), %edx	/* Move the 4 bytes of data at address ESI+four*EBX into EDX. */

Some examples of invalid accost calculations include:

mov (%ebx,%ecx,-1), %eax	/* Can only add register values. */
mov %ebx, (%eax,%esi,%edi,1)	/* At most ii registers in address ciphering. */

Functioning Suffixes

In general, the intended size of the of the data item at a given memory address can exist inferred from the assembly code didactics in which it is referenced. For example, in all of the above instructions, the size of the memory regions could be inferred from the size of the annals operand. When we were loading a 32-bit annals, the assembler could infer that the region of retentiveness 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 address to refer to a unmarried byte in retentivity.

Nonetheless, in some cases the size of a referred-to retentiveness region is ambiguous. Consider the instruction mov $two, (%ebx). Should this education move the value 2 into the single byte at address EBX? Possibly it should move the 32-bit integer representation of ii into the 4-bytes starting at address EBX. Since either is a valid possible interpretation, the assembler must exist explicitly directed as to which is right. The size prefixes b, w, and 50 serve this purpose, indicating sizes of 1, 2, and four bytes respectively.

For example:

movb $2, (%ebx)	/* Motion two into the single byte at the accost stored in EBX. */
movw $ii, (%ebx)	/* Movement the 16-scrap integer representation of ii into the two bytes starting at the accost in EBX. */
movl $2, (%ebx)	/* Movement the 32-bit integer representation of two into the iv bytes starting at the accost in EBX. */

Instructions

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

Nosotros use the following notation:

<reg32>	Any 32-fleck register (%eax, %ebx, %ecx, %edx, %esi, %edi, %esp, or %ebp)
<reg16>	Any 16-chip register (%ax, %bx, %cx, or %dx)
<reg8>	Any 8-flake annals (%ah, %bh, %ch, %dh, %al, %bl, %cl, or %dl)
<reg>	Any annals
<mem>	A retention accost (eastward.g., (%eax), 4+var(,1), or (%eax,%ebx,1))
<con32>	Any 32-fleck immediate
<con16>	Any 16-flake immediate
<con8>	Any 8-bit immediate
<con>	Whatsoever 8-, 16-, or 32-bit immediate

In assembly language, all the labels and numeric constants used every bit firsthand operands (i.e. not in an address calculation like 3(%eax,%ebx,8)) are always prefixed past a dollar sign. When needed, hexadecimal notation tin be used with the 0x prefix (eastward.grand. $0xABC). Without the prefix, numbers are interpreted in the decimal basis.

Data Motion Instructions

mov — Move

The mov instruction copies the data item referred to by its first operand (i.eastward. register contents, memory contents, or a constant value) into the location referred to by its 2d operand (i.e. a register or memory). While register-to-register moves are possible, straight memory-to-retentiveness moves are not. In cases where retentivity transfers are desired, the source retentivity contents must outset be loaded into a annals, and 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 — copy the value in EBX into EAX
movb $5, var(,1) — shop the value 5 into the byte at location var

push — Push on stack

The push teaching places its operand onto the top of the hardware supported stack in memory. Specifically, push beginning decrements ESP by 4, 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 downwardly — i.due east. the stack grows from high addresses to lower addresses.

Syntax
push button <reg32>
push <mem>
push button <con32>

Examples
button %eax — push eax on the stack
push var(,1) — push button the 4 bytes at address var onto the stack

pop — Pop from stack

The pop instruction removes the 4-byte data chemical element from the peak of the hardware-supported stack into the specified operand (i.eastward. annals or memory location). It start moves the iv bytes located at memory location (%esp) into the specified register or memory location, and and then increments ESP by 4.

Syntax
pop <reg32>
popular <mem>

Examples
pop %edi — pop the pinnacle element of the stack into EDI.
pop (%ebx) — popular the top chemical element of the stack into memory at the 4 bytes starting at location EBX.

lea — Load effective address

The lea educational activity places the accost specified by its showtime operand into the register specified by its second operand. Annotation, the contents of the retentivity location are not loaded, only the effective accost is computed and placed into the annals. This is useful for obtaining a arrow into a retentivity region or to perform unproblematic arithmetic operations.

Syntax
lea <mem>, <reg32>

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

Arithmetic and Logic Instructions

add together — Integer addition

The add instruction adds together its two operands, storing the effect in its second operand. Annotation, whereas both operands may be registers, at most one operand may exist a memory location.

Syntax
add <reg>, <reg>
add <mem>, <reg>
add <reg>, <mem>
add <con>, <reg>
add <con>, <mem>

Examples
add $10, %eax — EAX is set up to EAX + 10
addb $x, (%eax) — add together 10 to the unmarried byte stored at memory address stored in EAX

sub — Integer subtraction

The sub pedagogy stores in the value of its second operand the result of subtracting the value of its commencement operand from the value of its second operand. Every bit with add, whereas both operands may be registers, at almost one operand may be a memory location.

Syntax
sub <reg>, <reg>
sub <mem>, <reg>
sub <reg>, <mem>
sub <con>, <reg>
sub <con>, <mem>

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

inc, december — Increase, Decrement

The inc teaching increments the contents of its operand by ane. The december educational activity decrements the contents of its operand by one.

Syntax
inc <reg>
inc <mem>
dec <reg>
dec <mem>

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

imul — Integer multiplication

The imul pedagogy has two bones formats: ii-operand (first two syntax listings to a higher place) and three-operand (terminal two syntax listings in a higher place).

The two-operand form multiplies its ii operands together and stores the upshot in the 2nd operand. The event (i.e. second) operand must be a register.

The three operand course multiplies its second and third operands together and stores the event in its last operand. Over again, the result operand must be a register. Furthermore, the kickoff operand is restricted to being a abiding 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. Shop the result in EAX.

imul $25, %edi, %esi — ESI is fix 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 virtually meaning iv bytes and EAX every bit the to the lowest degree significant four bytes) by the specified operand value. The quotient result of the division is stored into EAX, while the remainder is placed in EDX.

Syntax
idiv <reg32>
idiv <mem>

Examples

idiv %ebx — split up the contents of EDX:EAX by the contents of EBX. Place the caliber in EAX and the residual in EDX.

idivw (%ebx) — dissever the contents of EDX:EAS by the 32-bit value stored at the memory location in EBX. Place the caliber in EAX and the residuum in EDX.

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

These instructions perform the specified logical performance (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 — clear all simply the last 4 bits of EAX.
xor %edx, %edx — gear up 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 $.25 of EAX

neg — Negate

Performs the ii's complement negation of the operand contents.

Syntax
neg <reg>
neg <mem>

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

shl, shr — Shift left and right

These instructions shift the $.25 in their first operand'southward 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 exist either an 8-fleck constant or the register CL. In either case, shifts counts of greater and 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 2 (if the most pregnant fleck is 0)

shr %cl, %ebx — Store in EBX the floor of event of dividing the value of EBX by 2 ⁿ where north is the value in CL. Caution: for negative integers, it is different from the C semantics of segmentation!

Control Flow Instructions

The x86 processor maintains an instruction pointer (EIP) annals that is a 32-chip value indicating the location in retentivity where the current instruction starts. Unremarkably, information technology increments to point to the next instruction in memory begins after execution an instruction. The EIP register cannot be manipulated directly, but is updated implicitly by provided control menstruation instructions.

We use the notation <label> to refer to labeled locations in the program text. Labels tin can be inserted anywhere in x86 assembly lawmaking text by entering a characterization proper name followed by a colon. For example,

            mov 8(%ebp), %esi brainstorm:        xor %ecx, %ecx        mov (%esi), %eax          

The second instruction in this code fragment is labeled begin. Elsewhere in the code, nosotros tin refer to the memory location that this didactics is located at in memory using the more than convenient symbolic name begin. This label is but a convenient way of expressing the location instead of its 32-bit value.

jmp — Leap

Transfers program control flow to the instruction at the memory location indicated by the operand.

Syntax
jmp <characterization>

Case
jmp begin — Bound to the education labeled begin.

jcondition — Conditional jump

These instructions are conditional jumps that are based on the status of a set of status codes that are stored in a special annals called the machine status word. The contents of the machine condition word include data nearly the last arithmetic functioning performed. For example, i bit of this word indicates if the last effect was zero. Some other indicates if the last consequence was negative. Based on these status codes, a number of conditional jumps can be performed. For example, the jz instruction performs a jump to the specified operand label if the result of the last arithmetics operation was cypher. Otherwise, control proceeds to the next instruction in sequence.

A number of the conditional branches are given names that are intuitively based on the last functioning performed being a special compare instruction, cmp (encounter below). For example, conditional branches such as jle and jne are based on kickoff performing a cmp operation on the desired operands.

Syntax
je <label> (jump when equal)
jne <characterization> (jump when not equal)
jz <label> (jump when concluding event was nada)
jg <label> (leap when greater than)
jge <characterization> (jump when greater than or equal to)
jl <label> (leap when less than)
jle <label> (bound 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, proceed to the next teaching.

cmp — Compare

Compare the values of the two specified operands, setting the condition codes in the machine status word appropriately. This instruction is equivalent to the sub instruction, except the effect of the subtraction is discarded instead of replacing the commencement operand.

Syntax
cmp <reg>, <reg>
cmp <mem>, <reg>
cmp <reg>, <mem>
cmp <con>, <reg>

Case
cmpb $10, (%ebx)
jeq loop

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

call, ret — Subroutine call and return

These instructions implement a subroutine phone call and return. The telephone call education first pushes the electric current code location onto the hardware supported stack in retentiveness (see the push instruction for details), and then performs an unconditional bound to the code location indicated by the label operand. Unlike the simple jump instructions, the phone call instruction saves the location to return to when the subroutine completes.

The ret instruction implements a subroutine render mechanism. This teaching first pops a code location off the hardware supported in-memory stack (encounter the popular didactics for details). Information technology and so performs an unconditional leap to the retrieved code location.

Syntax
call <label>
ret

Calling Convention

To allow separate programmers to share code and develop libraries for use by many programs, and to simplify the use of subroutines in general, programmers typically adopt a mutual calling convention. The calling convention is a protocol about how to phone call and return from routines. For case, given a set of calling convention rules, a programmer need not examine the definition of a subroutine to determine how parameters should be passed to that subroutine. Furthermore, given a set up of calling convention rules, high-level language compilers can be made to follow the rules, thus assuasive hand-coded associates language routines and loftier-level language routines to call one another.

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

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

The calling convention is broken into two sets of rules. The first fix of rules is employed past 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 exist used when implementing the phone call convention in your ain subroutines.

Stack during Subroutine Call

[Cheers to James Peterson for finding and fixing the problems in the original version of this figure!]

A good 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 prototype above depicts the contents of the stack during the execution of a subroutine with iii parameters and iii local variables. The cells depicted in the stack are 32-bit wide memory locations, thus the retention addresses of the cells are 4 bytes apart. The first parameter resides at an offset of 8 bytes from the base arrow. To a higher place the parameters on the stack (and below the base pointer), the phone call didactics placed the render address, thus leading to an extra 4 bytes of offset from the base arrow to the first parameter. When the ret instruction is used to return from the subroutine, it will jump to the return accost stored on the stack.

Caller Rules

To make a subrouting phone call, the caller should:

1. Before calling a subroutine, the caller should save the contents of sure 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 after the subroutine returns, the caller must push the values in these registers onto the stack (then they can exist restore after the subroutine returns.
2. To pass parameters to the subroutine, push button them onto the stack earlier the call. The parameters should be pushed in inverted order (i.e. final parameter commencement). Since the stack grows down, the commencement parameter will be stored at the lowest address (this inversion of parameters was historically used to allow functions to be passed a variable number of parameters).
3. To phone call the subroutine, employ the telephone call instruction. This educational activity places the render accost on elevation of the parameters on the stack, and branches to the subroutine code. This invokes the subroutine, which should follow the callee rules beneath.

After the subroutine returns (immediately post-obit the call instruction), the caller can expect to find the render value of the subroutine in the register EAX. To restore the car state, the caller should:

1. Remove the parameters from stack. This restores the stack to its country earlier the call was performed.
2. Restore the contents of caller-saved registers (EAX, ECX, EDX) by popping them off of the stack. The caller can presume that no other registers were modified by the subroutine.

Instance

The code below shows a function call that follows the caller rules. The caller is calling a function myFunc that takes three integer parameters. First parameter is in EAX, the second parameter is the constant 216; the third parameter is in the retention location stored in EBX.

push button (%ebx)    /* Push last parameter first */ push $216      /* Push the second parameter */ push %eax      /* Push beginning parameter concluding */  call myFunc    /* Call the function (assume C naming) */  add together $12, %esp          

Note that subsequently the call returns, the caller cleans upward the stack using the add pedagogy. Nosotros have 12 bytes (iii parameters * 4 bytes each) on the stack, and the stack grows downwardly. Thus, to get rid of the parameters, we 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 take been changed. If the caller uses them after the call, it 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 attach to the following rules at the beginning of the subroutine:

1. Push the value of EBP onto the stack, then copy the value of ESP into EBP using the following instructions:

                 button %ebp     mov  %esp, %ebp            

   This initial action maintains the base pointer, EBP. The base pointer is used by convention as a point of reference for finding parameters and local variables on the stack. When a subroutine is executing, the base of operations pointer holds a copy of the stack pointer value from when the subroutine started executing. Parameters and local variables will always exist located at known, constant offsets away from the base arrow value. Nosotros button the onetime base arrow value at the starting time of the subroutine and then that we can later restore the advisable base arrow value for the caller when the subroutine returns. Call back, the caller is not expecting the subroutine to change the value of the base of operations pointer. We so motility the stack pointer into EBP to obtain our point of reference for accessing parameters and local variables.
2. Next, allocate local variables by making space on the stack. Recall, the stack grows downwardly, so to brand space on the meridian of the stack, the stack pointer should be decremented. The corporeality by which the stack pointer is decremented depends on the number and size of local variables needed. For example, 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.eastward., sub $12, %esp). Every bit with parameters, local variables will exist located at known offsets from the base pointer.
3. Next, salve the values of the callee-saved registers that will exist used by the function. To salve registers, push them onto the stack. The callee-saved registers are EBX, EDI, and ESI (ESP and EBP will also be preserved past the calling convention, but need not be pushed on the stack during this footstep).

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

1. Leave the return value in EAX.
2. Restore the former values of whatsoever callee-saved registers (EDI and ESI) that were modified. The register contents are restored by popping them from the stack. The registers should be popped in the inverse guild that they were pushed.
3. Deallocate local variables. The obvious way to exercise 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 motility the value in the base pointer into the stack arrow: mov %ebp, %esp. This works considering the base of operations arrow ever contains the value that the stack pointer contained immediately prior to the allocation of the local variables.
4. Immediately before returning, restore the caller'south base of operations pointer value by popping EBP off the stack. Think that the showtime affair we did on entry to the subroutine was to push the base pointer to save its erstwhile value.
5. Finally, return to the caller by executing a ret teaching. This instruction will detect and remove the appropriate return address from the stack.

Note that the callee'due south rules fall cleanly into two halves that are basically mirror images of one some other. The first one-half of the rules apply to the get-go of the function, and are commonly said to define the prologue to the function. The latter one-half of the rules use to the end of the function, and are thus unremarkably said to define the epilogue of the function.

Instance

Hither is an example part definition that follows the callee rules:

            /* Start the code department */   .text    /* Define myFunc as a global (exported) role. */   .globl myFunc   .type myFunc, @function myFunc:    /* Subroutine Prologue */   button %ebp      /* Relieve the old base of operations arrow value. */   mov %esp, %ebp /* Set the new base arrow value. */   sub $4, %esp   /* Make room for 1 4-byte local variable. */   push %edi      /* Salve the values of registers that the function */   push %esi      /* will modify. This function uses EDI and ESI. */   /* (no need to save EBX, EBP, or ESP) */    /* Subroutine Body */   mov viii(%ebp), %eax   /* Move value of parameter ane into EAX. */   mov 12(%ebp), %esi  /* Move value of parameter ii into ESI. */   mov 16(%ebp), %edi  /* Move value of parameter iii into EDI. */    mov %edi, -four(%ebp)  /* Move EDI into the local variable. */   add together %esi, -four(%ebp)  /* Add together ESI into the local variable. */   add together -4(%ebp), %eax  /* Add the contents of the local variable */                       /* into EAX (final consequence). */    /* Subroutine Epilogue */   pop %esi       /* Recover register values. */   pop %edi   mov %ebp, %esp /* Deallocate the local variable. */   pop %ebp       /* Restore the caller's base of operations 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 past decrementing the stack pointer, and saving register values on the stack.

In the body of the subroutine we can see the use of the base pointer. Both parameters and local variables are located at abiding offsets from the base pointer for the duration of the subroutines execution. In particular, we discover that since parameters were placed onto the stack before the subroutine was chosen, they are always located below the base arrow (i.due east. at college addresses) on the stack. The first parameter to the subroutine can always be constitute at memory location (EBP+8), the second at (EBP+12), the tertiary at (EBP+16). Similarly, since local variables are allocated after the base of operations pointer is set, they always reside higher up the base pointer (i.e. at lower addresses) on the stack. In particular, the first local variable is always located at (EBP-four), the 2nd at (EBP-8), and so on. This conventional use of the base pointer allows us to speedily place the use of local variables and parameters inside a function body.

The role epilogue is basically a mirror image of the role prologue. The caller'southward 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 render to the appropriate code location in the caller.

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

Source: https://flint.cs.yale.edu/cs421/papers/x86-asm/asm.html

Posted by: smithtwen1937.blogspot.com

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