Sum of an array

This program adds six qwords by walking from the address of the first one to the address just after the last one. rsi holds the address to read next, rdi holds the stopping address, and rax holds the running sum.

default rel
global _start

section .data
numbers:    dq 4, 8, 15, 16, 23, 42
numbers_end:

section .text
_start:
    lea rsi, [numbers]      ; address of the first element
    lea rdi, [numbers_end]  ; address just after the last element
    xor rax, rax            ; running sum = 0
.next:
    add rax, [rsi]          ; add the qword at address rsi
    add rsi, 8              ; advance by one qword
    cmp rsi, rdi
    jb .next                ; repeat while rsi is below the end
    mov r8, rax             ; save the sum for inspection
    mov r9, rdi             ; save the end address for inspection

    mov rax, 60
    xor rdi, rdi
    syscall

The brackets in [rsi] mean “the qword stored at the address in rsi.” On the first pass, the loop adds 4 to rax and moves rsi from numbers to numbers + 8. Since that address is still below numbers_end, jb takes the loop back. On the second pass, it adds 8, making rax = 12, then moves rsi to numbers + 16. The same two instructions read and advance through the remaining qwords.

numbers_end: names the address where the next qword would begin. The label itself stores no data; the assembler assigns it an address after the six qwords. Six qwords occupy 48 bytes, so after the last addition rsi has advanced 48 bytes from numbers and equals rdi. The jb condition is then false. The program compares with this end address but never reads from it.

Run the program and inspect the registers. r8 holds the answer, 108: the program copies the sum there before setting rax to 60 for the exit syscall. r9 saves the end address before the exit setup clears rdi. At the end, rsi and r9 both show 0x402030, 48 bytes past numbers at 0x402000. This loop shape assumes the array has at least one qword, as it reads before checking whether it has reached the end.

Your turn

Add 100 as a seventh qword in the dq list, then complete the missing instruction so the loop adds each qword to rax. Keep numbers_end: immediately after the data. Before running, predict the sum; then check that r8 is 208 and rsi equals the saved end address in r9. Use Test to check the sum.

default rel
global _start

section .data
numbers:    dq 4, 8, 15, 16, 23, 42  ; add 100 here
numbers_end:

section .text
_start:
    lea rsi, [numbers]
    lea rdi, [numbers_end]
    xor rax, rax
.next:
    ; Add the qword at rsi to rax.
    add rsi, 8
    cmp rsi, rdi
    jb .next
    mov r8, rax
    mov r9, rdi

    mov rax, 60
    xor rdi, rdi
    syscall

default rel
global _start

section .data
numbers:    dq 4, 8, 15, 16, 23, 42, 100
numbers_end:

section .text
_start:
    lea rsi, [numbers]
    lea rdi, [numbers_end]
    xor rax, rax
.next:
    add rax, [rsi]
    add rsi, 8
    cmp rsi, rdi
    jb .next
    mov r8, rax
    mov r9, rdi

    mov rax, 60
    xor rdi, rdi
    syscall