generate x86-64 assembly to compute the expression
a*b + c*d

the final result should be stored in %rax

a is in %rcx
b is in %rdx
c is in %rsi
d is in %rdi

it should take no more than 4 instructions

imulq %rdx, %rcx
imulq %rdi, %rsi
addq %rsi, %rcx
movq %rcx, %rax
or
imulq %rdx, %rcx
imulq %rdi, %rsi
leaq (%rsi, %rcx), %rax

what is a C expression equivalent to the following assembly
sub %rsi,%rdi
mov %rdi,%rax
shl $0x4,%rax
add %rdi,%rax

%rsi holds a variable b
%rdi holds a variable a

17 * (a - b)

jumps

jump instructions are how things like ifs and loops are achieved in assembly

a jump instruction takes as an argument a memory address and then may set the instruction pointer to that address

most jump instructions depend on the value of one or more condition codes

list of jump instructions: http://courses.cs.washington.edu/courses/cse374/16wi/lectures/jumping.pdf

condition codes

processor has 1-bit flags that get set based on the result of arithmetic instructions

carry flag (CF) set if carry out from most significant bit

used for unsigned

zero flag (ZF) set if the result is zero

sign flag (SF) set if the result is negative

overflow flag (OF) set if there is two’s complement (signed) overflow

for addq

operands are positive, result is negative

operands are negative, result is positive

how instructions set condition codes: http://courses.cs.washington.edu/courses/cse374/16wi/lectures/condition-codes.pdf

example

function with an if statement

-Og vs -O1 vs -O2

see shell recording: http://courses.cs.washington.edu/courses/cse374/16wi/lectures/21-shell_recording.html

aside: can refer to lower-order bits of registers by using different names

for example, %eax is the lower 32 bits of register %rax

diagram: http://courses.cs.washington.edu/courses/cse374/16wi/lectures/x86-64_registers.png

note the uses for each register

int fact(int n) {
int r = 1;
while(n > 1)
r *= n;
n--;
}
return r;
}

0000000000400616 <fact>:
400616: b8 01 00 00 00 mov $0x1,%eax
40061b: eb 06 jmp 400623 <fact+0xd>
40061d: 0f af c7 imul %edi,%eax
400620: 83 ef 01 sub $0x1,%edi
400623: 83 ff 01 cmp $0x1,%edi
400626: 7f f5 jg 40061d <fact+0x7>
400628: f3 c3 retq

trace the series of instructions, register values, and condition codes for fact(3)

http://courses.cs.washington.edu/courses/cse374/16wi/lectures/factorial-example.md.html