.data
    buf:
    .byte   0

.text
.type fun,@function
.global fun

#rdi - pierwszy argument
#rsi - drugi argument
#rdx - trzeci argument

fun:
    MOV %rdi, buf

nextChar:
    JMP takeFromFirst
    JMP takeFromSecond
    JMP nextChar

takeFromFirst:
    MOV(%rsi), %al
    CMPB $0, %al
    JE isEndOfSecond
    MOV %al, (%rdi)
    INC %rdi
    INC %rsi

takeFromSecond:
    MOV(%rdx), %al
    CMPB $0, %al
    JE isEndOfFirst
    MOV %al, (%rdi)
    INC %rdi
    INC %rdx

isEndOfFirst:
    MOV (%rsi),%al
    CMPB $0, %al
    JE exit
    JMP takeFromFirst

isEndOfSecond:
    MOV (%rdx),%al
    CMPB $0, %al
    JE exit
    JMP takeFromSecond

exit:
    MOV buf, %rax
    RET