You are to write a C program called big_mult.c that multiplies two unsigned 64-bit integers, x and y, read from the command line. The output is a pair of unsigned 64-bit integers representing the most significant and least significant 64 bits of the full 128-bit product x * y. The inputs and outputs are to be given in hexadecimal format. Your C program will take care of reading the inputs and printing the output, but it will call a function mulq.s to do the actual multiplication. Your C program should use only unsigned long long int variables and should not do any arithmetic. To reduce the length of our type declarations, I put the following lines into hw3.h.
typedef unsigned long long int ulli;
typedef long long int lli;
This allows us to abbreviate unsigned long long int with the shorter name ulli and long long int with lli.
The function defined by mulq.s should have the following declaration in C before the function main.
void mulq(ulli x, ulli y, ulli *high, ulli *low);
The least significant 64 bits of the product are to be assigned to low, and the most significant 64 bits of the product are to be assigned to high. Remember to put an appropriate header comment into your assembly file (the ordinary C comment /* ... */ will work for assembly too). You will also need to read carefully the description of the mulq instruction in the Intel 64/IA32 instruction set reference manual, Intel 64/IA32 instruction set reference manual, or equivalent documentation. Learning to read this two-volume 800+ page document is part of your learning experience on this assignment.
One way to approach writing this assembly program is to write a similar program in C, compile it to assembly code using the -S option, and modify the resulting assembly code to do what you need. Your final assembly code should be very short and should contain only one multiplication instruction mulq.
The compile command to test your programs will look like this:
gcc64 -Wall -std=gnu99 -o big_mult big_mult.c mulq.s .
Here are 32- and 64-bit sample outputs to use in testing your programs.
C:>big_mult 2f432f43 629b03cb
2f432f43 x 629b03cb = 12345678 87654321
C:>big_mult 99d0c486a0fad481 76a185cea6f497c7
99d0c486a0fad481 x 76a185cea6f497c7 = 4747474747474747 4747474747474747
.
Remember that arguments are passed differently in the 64-bit architecture than in the 32-bit architecture. The registers used for parameter passing in Windows are different from those described in the textbook for Unix. Windows passes only the first four parameters in registers. The registers used by Unix are %rdi, %rsi, %rdx, %rcx, %r8, %r9 in that order. The registers used by Windows are %rcx, %rdx, %r8, %r9 . See Class10.pdf for details on register usage.