X86-64 Architecture Guide

Reference

This handout only mentions a small subset of the rich possibilities provided by the x86-64 instruction set and architecture. For a more complete (but still readable) introduction, consult The AMD64 Architecture Programmer’s Manual, Volume 1: Application Programming.

Registers

In the assembly syntax accepted by gcc, register names are always prefixed with %. All of these registers are 64 bits wide.

The register file is as follows:

Register	Purpose	Saved across calls
%rax	temp register; return value	No
%rbx	callee-saved	Yes
%rcx	used to pass 4th argument to functions	No
%rdx	used to pass 3rd argument to functions	No
%rsp	stack pointer	Yes
%rbp	callee-saved; base pointer	Yes
%rsi	used to pass 2nd argument to functions	No
%rdi	used to pass 1st argument to functions	No
%r8	used to pass 5th argument to functions	No
%r9	used to pass 6th argument to functions	No
%r10-r11	temporary	No
%r12-r15	callee-saved registers	Yes

For the code generation phase of the project you will not be performing register allocation. You should use %r10 and %r11 for temporary values that you load from the stack.

Instruction Set

Each mnemonic opcode presented here represents a family of instructions. Within each family, there are variants which take different argument types (registers, immediate values, or memory addresses) and/or argument sizes (byte, word, double-word, or quad-word). The former can be distinguished from the prefixes of the arguments, and the latter by an optional one-letter suffix on the mnemonic.

For example, a mov instruction which sets the value of the 64-bit %rax register to the immediate value 3 can be written as

movq $3, %rax

Immediate operands are always prefixed by $. Un-prefixed operands are treated as memory addresses, and should be avoided since they are confusing.

For instructions which modify one of their operands, the operand which is modified appears second. This differs from the convention used by Microsoft’s and Borland’s assemblers, which are commonly used on DOS and Windows.

Opcode	Description
Copying values
mov src, dest	Copies a value from a register, immediate value or memory address to a register or memory address.
cmove %src, %dest	Copies from register %src to register %dest if the last comparison operation had the corresponding result (cmove: equality, cmovne: inequality, cmovg: greater, cmovl: less, cmovge: greater or equal, cmovle: less or equal).
cmovne %src, %dest
cmovg %src, %dest
cmovl %src, %dest
cmovge %src, %dest
cmovle %src, %dest
Stack management
enter $x, $0	Sets up a procedure’s stack frame by first pushing the current value of %rbp on to the stack, storing the current value of %rsp in %rbp, and finally decreasing %rsp to make room for x byte-sized local variables.
leave	Removes local variables from the stack frame by restoring the old values of %rsp and %rbp.
push src	Decreases %rsp and places src at the new memory location pointed to by %rsp. Here, src can be a register, immediate value or memory address.
pop dest	Copies the value stored at the location pointed to by %rsp to dest and increases %rsp. Here, dest can be a register or memory location.
Control flow
call target	Jump unconditionally to target and push return value (current PC + 1) onto stack.
ret	Pop the return address off the stack and jump unconditionally to this address.
jmp target	Jump unconditionally to target, which is specified as a memory location (for example, a label).
je target	Jump to target if the last comparison had the corresponding result (je: equality; jne: inequality).
jne target
Arithmetic and logic
add src, dest	Add src to dest.
sub src, dest	Subtract src from dest.
imul src, dest	Multiply dest by src.
idiv divisor	Divide rdx:rax by divisor. Store quotient in rax and store remainder in rdx.
shr reg	Shift reg to the left or right by value in cl (low 8 bits of rcx).
shl reg
ror src, dest	Rotate dest to the left or right by src bits.
cmp src, dest	Set flags corresponding to whether dest is less than, equal to, or greater than src

Stack Organization

Global and local variables are stored on the stack, a region of memory that is typically addressed by offsets from the registers %rbp and %rsp. Each procedure call results in the creation of a stack frame where the procedure can store local variables and temporary intermediate values for that invocation.The stack is organized as follows:

Position	Contents	Frame
8n+16(%rbp)	argument n	Previous
...	...
16(%rbp)	argument 7
8(%rbp)	return address	Current
0(%rbp)	previous %rbp value
-8(%rbp)	locals and temps
...
0(%rsp)

Calling Convention

We will use the standard Linux function calling convention. The calling convention is defined in detail in System V Application Binary Interface—AMD64 Architecture Processor Supplement. We will summarize the calling convention as it applies to decaf. The caller uses registers to pass the first 6 arguments to the callee. Given the arguments in left-to-right order, the order of registers used is: %rdi, %rsi, %rdx, %rcx, %r8, and %r9. Any remaining arguments are passed on the stack in reverse order so that they can be popped off the stack in order. The callee is responsible for perserving the value of registers %rbp %rbx, and %r12-r15, as these registers are owned by the caller. The remaining registers are owned by the callee.

The callee places its return value in %rax and is responsible for cleaning up its local variables as well as for removing the return address from the stack.

The call, enter, leave and ret instructions make it easy to follow this calling convention.

Since we follow the standard linux ABI, we can call C functions and library functions using our callout structure. For the purposes of the project we are only going to call printf and get_int_035. When calling printf, we must set the value of register %rax to 0 before issuing the call instruction. This is because printf uses a variable number of arguments and %rax specifies how many SSE registers are used for the arguments. For our purposes the value will always be 0. Since callouts can only return an single integer value, we have provided a function get_int_035(), which will read a single integer input from the terminal and return its integer value. This function is included in the 6035 static library. We cannot use scanf because it returns the number of items read.