2017/Asm1

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Assembly 1

Registers

x86-64 has 14 general-purpose registers and several special-purpose registers. This table gives all the basic registers, with special-purpose registers highlighted in yellow. You’ll notice different naming conventions, a side effect of the long history of the x86 architecture (the 8086 was first released in 1978).

Register 32-bit
(bits 0–31)
16-bit
(bits 0–15)
8-bit low
(bits 0–7)
8-bit high
(bits 8–15)
Calling convention/Name Callee-saved?
%rax  %eax  %ax  %al  %ah Return value
(Accumulator)
No
%rbx  %ebx  %bx  %bl  %bh Yes
%rcx  %ecx  %cx  %cl  %ch 4th parameter No
%rdx  %edx  %dx  %dl  %dh 3rd parameter No
%rsi  %esi  %si  %sil 2nd parameter No
%rdi  %edi  %di  %dil 1st parameter No
%rbp  %ebp  %bp  %bpl Base pointer Yes
%rsp  %esp  %sp  %spl Stack pointer Yes
%r8  %r8d  %r8w  %r8b 5th parameter No
%r9  %r9d  %r9w  %r9b  – 6th parameter No
%r10  %r10d  %r10w  %r10b No
%r11  %r11d  %r11w  %r11b No
%r12  %r12d  %r12w  %r12b Yes
%r13  %r13d  %r13w  %r13b Yes
%r14  %r14d  %r14w  %r14b Yes
%r15  %r15d  %r15w  %r15b Yes
%rip
($pc in GDB)
 %eip  %ip –  Instruction pointer
(Program counter)
*
%rflags  %eflags  %flags Flags and condition codes No

The %rbp register has a special purpose: it points to the bottom of the current function’s stack frame, and local variables are often accessed relative to its value. However, when optimization is on, the compiler may determine that all local variables can be stored in registers. This frees up %rbp for use as another general-purpose register.

The relationship between different register bit widths is a little weird.

  1. Loading a value into a 32-bit register name sets the upper 32 bits of the register to zero. Thus, after movl $-1, %eax, the %rax register has value 0x00000000FFFFFFFF.
  2. Loading a value into a 16- or 8-bit register name leaves all other bits unchanged.

There are special instructions for loading signed and unsigned 8-, 16-, and 32-bit quantities into registers, recognizable by instruction suffixes. For instance, movzbl moves an 8-bit quantity (a byte) into a 32-bit register (a longword) with zero extension; movslq moves a 32-bit quantity (longword) into a 64-bit register (quadword) with sign extension. There’s no need for movzlq (why?).

Instruction format

The basic kinds of assembly instructions are:

  1. Computation. These instructions perform computation on values stored in registers. Most have zero or one source registers and one source/destination register. The source register, if any, comes first. For example, the instruction addq %rax, %rbx performs the computation %rbx := %rbx + %rax.
  2. Data movement. These instructions move data between registers and memory. Almost all have one source register and one destination register; the source register comes first.
  3. Control flow. Normally the CPU executes instructions in sequence. Control flow instructions change the instruction pointer in other ways. There are unconditional branches (the instruction pointer is set to a new value), conditional branches (the instruction pointer is set to a new value if a condition is true), and function call and return instructions.

(We use the “AT&T syntax” for x86-64 assembly. For the “Intel syntax,” which you can find in online documentation from Intel, see the Aside in CS:APP3e §3.3, p177, or Wikipedia, or other online resources. AT&T syntax is distinguished by several features, but especially by the use of percent signs for registers. Sadly, the Intel syntax puts destination registers before source registers.)

Some instructions appear to combine computation and data movement. For example, given the C code int* ip; ... ++(*ip); the compiler might generate incl (%rax) rather than movl (%rax), %ebx; incl %ebx; movl %ebx, (%rax). However, such mixed instructions are actually executed as distinct phases: data movement, then computation, then data movement. This matters when we introduce parallelism.

Directives

Assembly code interleaves instructions with labels and directives. Labels look like labelname: or labelnumber:; directives look like .directivename arguments. Labels are markers in the generated assembly, used to compute addresses; we usually see them used in control flow instructions. Directives are instructions to the assembler; for instance, the .globl L instruction says “label L is globally visible in the executable”, .align sets the alignment of the following data, .long puts a number in the output, and .text and .data define the current segment.

Stack

The architecture has special support for the stack segment thanks to the push, pop, call, and ret instructions.

A push instruction pushes a value onto the stack. This both modifies the stack pointer (making it smaller) and modifies the stack segment (by moving data there). For instance, the instruction pushq X means:

subq $8, %rsp
movq X, (%rsp)

And popq X undoes the effect of pushq X. It means:

movq (%rsp), X
addq $8, %rsp

X can be a register or a memory reference. Since pushing things onto the stack causes the stack pointer’s address value to shrink, we often say that the “stack grows down.”

callq ADDR has the effect of these pseudo-instructions: pushq [NEXT INSTRUCTION POINTER]; j ADDR.

retq has the effect of this pseudo-instruction: popq %rip.

Address modes

Instruction arguments use the following syntax. (See also CS:APP3e Figure 3.3 in §3.4.1, p181.)

Type Example syntax Value used
Register %rbp Contents of %rbp
Immediate $0x4 0x4
$symbol_name Address of global symbol_name
Memory 0x4 Value at address 0x4
symbol_name Value at address of global symbol_name
(%rax) Value located at address in %rax
0x4(%rax) Value located at address %rax + 4
(%rax,%rbx) Value located at address %rax + %rbx
(%rax,%rbx,4) Value located at address %rax + %rbx*4
0x18(%rax,%rbx,4) Value located at address %rax + 0x18 + %rbx*4

In the full form 0x18(%rax,%rbx,4), %rax is the base, 0x18 the offset, %rbx the index, and 4 the scale. The scale can be either 1, 2, 4, or 8.

x86-64 code often refers to globals using %rip-relative addressing; you might see a(%rip) instead of just plain a. This style of global reference supports features like position-independent code (e.g., shared libraries) and address space layout randomization (improves security).

Conditional branches

Arithmetic instructions change part of the %rflags register as a side effect of their operation. The most often used flags are:

  • ZF (zero flag): set iff the result was zero.
  • SF (sign flag): set iff the most significant bit (the sign bit) of the result was one (i.e., the result was negative if considered as a signed integer).
  • CF (carry flag): set iff the result overflowed when considered as unsigned (i.e., the result was greater than 2W-1).
  • OF (overflow flag): set iff the result overflowed when considered as signed (i.e., the result was greater than 2W-1-1 or less than –2W-1).

Although some instructions let you load specific flags into registers (e.g., setz; see CS:APP3e §3.6.2, p203), code more often accesses them via conditional jump or conditional move instructions.

Instruction Mnemonic C example Flags
j (jmp) Jump break; (Unconditional)
je (jz) Jump if equal (zero) if (x == y) ZF
jne (jnz) Jump if not equal (nonzero) if (x != y)  !ZF
jg (jnle) Jump if greater if (x > y), signed  !ZF && !(SF ^ OF)
jge (jnl) Jump if greater or equal if (x >= y), signed  !(SF ^ OF)
jl (jnge) Jump if less if (x < y), signed SF ^ OF
jle (jng) Jump if less or equal if (x <= y), signed (SF ^ OF) || ZF
ja (jnbe) Jump if above if (x > y), unsigned  !CF && !ZF
jae (jnb) Jump if above or equal if (x >= y), unsigned  !CF
jb (jnae) Jump if below if (x < y), unsigned CF
jbe (jna) Jump if below or equal if (x <= y), unsigned CF || ZF
js Jump if sign bit if (x < 0), signed SF
jns Jump if not sign bit if (x >= 0), signed  !SF

The test and cmp instructions are frequently seen before a conditional branch. These operations perform arithmetic but throw away the result, except for condition codes. test performs binary-and, cmp performs subtraction. cmp is hard to grasp: remember that subq %rax, %rbx performs %rbx := %rbx - %rax—the source/destination operand is on the left. So cmpq %rax, %rbx evaluates %rbx - %rax. The sequence cmpq %rax, %rbx; jg L will jump to label L if %rbx is greater than %rax (signed).