The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.

Find the sum of all the primes below two million.

.syntax unified .equ word,4 .equ limit,2000000 .equ numprimes4,595732 @ sufficient to hold all the bytes in all the elements of the vector we use to hold the primes sum_hi .req r4 sum_lo .req r5 numprimes .req r6 primes_ptr .req r7 number .req r8 limit .req r9 .section .bss .lcomm primes_vector,numprimes4 @ allocate space for vector of primes .section .rodata llustring: .asciz "%llu\n" .text .align 8 .global main .type main, %function main: stmfd sp!, {r4-r9, fp, lr} ldr primes_ptr, =primes_vector mov numprimes, 1 mov number, 2 str number, [primes_ptr] @ store 2 to primes vector ldr limit, =limit mov sum_hi, 0 @ initialise sum mov sum_lo, 2 mov number, 3 @ start with 3 as first odd prime loop: cmp number, limit bge printme @ leave if we have exceeded limit mov r0, number ldr r1, =primes_vector mov r2, numprimes bl prime_vector teq r0, 1 @ check for primality of number bne nexti str number, [primes_ptr, numprimes, lsl 2] @ if prime, add to vector add numprimes, numprimes, 1 @ increment prime count adds sum_lo, sum_lo, number @ and add to sum adc sum_hi, sum_hi, 0 nexti: add number, number, 2 @ increment number by 2 b loop printme: mov r2, sum_lo mov r3, sum_hi ldr r0, =llustring @ store address of start of string to r0 bl printf mov r0, 0 ldmfd sp!, {r4-r9, fp, pc} mov r7, 1 @ set r7 to 1 - the syscall for exit swi 0 @ then invoke the syscall from linux

.syntax unified .equ word, 4 # this subroutine returns 1 if the passed number is prime; 0 if not # # inputs # r0 - integer to test # r1 - pointer to vector of prime integers smaller than r0 # r2 - length of vector passed in r1 # # outputs # r0 - prime boolean number .req r4 vptr .req r5 tmp .req r6 squared .req r7 vsize .req r8 .global prime_vector .type prime_vector, %function .text prime_vector: stmfd sp!, {r4-r8, lr} mov number, r0 mov vptr, r1 mov vsize, r2 nexti: ldr tmp, [vptr], word @ get vector element to tmp mul squared, tmp, tmp @ and square it cmp squared, number @ compare the square to the input number movgt r0, 1 @ store 1 to r0 if square is greater (i.e. number is primes) bgt last @ and jump out mov r0, number mov r1, tmp bl divide @ divide number by tmp teq r1, 0 @ and check if it is a divisor moveq r0, 0 @ if so, not prime, so set r0 to 0 beq last @ and leave subs vsize, vsize, 1 @ decrement prime counter bgt nexti @ and try next vector element mov r0, 1 last: ldmfd sp!, {r4-r8, pc}

prime_vector.s

.syntax unified # divide takes value in r0, divisor in r1 and returns dividend in r0 and modulus in r1 .global divide .type divide, %function divide: stmfd sp!, {lr} # see http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf bl __aeabi_uidivmod ldmfd sp!, {pc}

divide.s

## No comments:

## Post a Comment