index.md (1593B)
1 +++ 2 title = 'Subroutines & the stack' 3 +++ 4 # Subroutines & the stack 5 ## Subroutines 6 7 allow a block of instructions to be called any number of times without having to rewrite it 8 9 calling a subroutine: 10 1. Store contents of PC in link register (contains return address) 11 2. Branch to target address specified by call instruction 12 3. Execute whatever code is there 13 4. When return is called, branch to address in link register 14 15 when nesting, the return address is pushed to the stack by the caller. on return the callee pops the saved return address from the stack into the link register. 16 17 parameters can be passed by: 18 19 - registers — convenient & efficient, but may not be enough registers 20 - processor stack — can pass an arbitrary amount of parameters 21 22 ## Stack 23 a pile of stuff where the stuff can only be added to or taken from the top 24 25 also called LIFO (last-in-first-out) 26 27 push (add elements) 28 1. subtract 4 from SP (32-bit) 29 2. move value from register to address stored in SP 30 31 pop (remove elements) 32 1. move value from byte at address stored in SP to register 33 2. add 4 to SP (32-bit) 34 35 processor stack is used for storing data, e.g. parameters, registers 36 37 stack pointer register (SP) holds address of top element in stack 38 39 ‘grows downwards’ — from high to low addresses, decreasing size 40 41 stack fame — ‘private workspace’ for a subroutine 42 43 - allocated at start, deallocated at end of subroutine 44 - can also be used for local memory variables 45 - base/frame pointer points to base of current frame, can easily access parameters by using offset(%rbp) 46 47 