Stack, subprograms. procedures and modular programming role of stack while using procedures stack implementation (Pentium)

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1 Assembler lecture 3 S.Šimoňák, DCI FEEI TU of Košice Stack, subprograms procedures and modular programming role of stack while using procedures stack implementation (Pentium) Stack LIFO data structure, operations PUSH and POP directly accessible only element at the top (TOS) the order of inserted/reed elements Stack implementation (Pentium) memory area reserved in the stack segment top of the stack (TOS) given by SS:ESP SS stack segment, ESP relative address (offset) word (16-bit), doubleword (32-bit) can be stored stack grow direction - to lower addresses TOS last item inserted (lower byte) Example: stack of size 256B a) empty attempt to POP error (stack underflow) b) word PUSHed ESP decreased, then stored (little endian) c) doubleword PUSHed (4B) stack full (ESP=0), attempt to PUSH error (stack overflow) POPping the stack ESP increased (memory without changes) freed memory cells usable for storing new values

2 Stack operations instructions push and pop storing word/doubleword onto stack syntax push src (src 16/32-bit GPR, SR, memory, constant) pop dst (dst - 16/32-bit GPR, SR, memory) semantics push src16 push src32 pop dst16 pop dst32 ESP ESP 2, [SS:ESP] src16 ESP ESP 4, [SS:ESP] src32 dst16 [SS:ESP], ESP ESP+2 dst32 [SS:ESP], ESP ESP+4 instructions pushf and popf storing and restoring the flag register (FLAGS) syntax pushfd (32-bit FLAGS) pushfw (16-bit FLAGS) popfd popfw instructions pusha and popa storing GPR register (8) syntax pushad popad pushaw popaw (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI) (restores, except the ESP) (AX, CX, DX, BX, SP, BP, SI, DI) (except the SP)

3 Typical stack usage temporary data storage limited number of GPR registers storing and restoring contents of registers Example: exchange of contents of two 32-bit variables (value1, value2) a) push EAX ; storing b) push value1 push EBX push value2 EAX, value1 pop value1 EBX, value2 pop value2 value1, EBX value2, EAX pop EBX pop EAX ; restoring the contents program execution control ing procedures (urn address) passing parameters to procedures (parameter passing) HL languages Procedures logiy standalone part of code for performing given task mechanisms of parameter passing by value actual values of parameters only (without changes by procedure) by reference addresses of parameters (change of parameter values possible) HLL (2 types of subprograms) functions (only one urn value) procedures (no/more than one urn values)

4 Procedures in x86 language (Pentium) ing the procedure (proc-name relative to the instruction following the (offset)) Example [1]: proc-name (ESP ESP-4, [SS:ESP] EIP, EIP EIP+rel.offset) after the fetch, EIP = H storing EIP into stack transfer of control to start of 'sum' procedure adding the offset to the EIP H H = DH forward positive offset (signed, 32-bit) urn from the procedure control passing back from ee to er procedure (instruction following the ) urn address (stack) (EIP [SS:ESP], ESP ESP+4)

5 parameter of (number of bytes to free from the stack, optional) n Parameter passing storing parameters to given place (registers, stack) ing the procedure Register method GPR registers Example: by value, register method %include "asm_io.inc" segment.data prompt_msg1 db "Enter first number: ",0 prompt_msg2 db "Enter second number: ",0 sum_msg db "The sum is: ",0 segment.text global _asm_main _asm_main: enter 0,0 pusha EAX, prompt_msg1 print_string read_int ECX, EAX print_nl EAX, prompt_msg2 sum: print_string read_int EDX, EAX print_nl EAX, sum_msg print_string sum print_int print_nl popa EAX, 0 leave add EAX,ECX EAX,EDX

6 register method properties advantages simpler for small number of parameters fast (parameters in registers) disadvantages limited number of GPR registers (parameters) often temporary storage of registers in use is required (stack) lost of the speed advantage Stack method all parameters on the stack before the procedure problem access to parameters (/EIP) push push number1 number2 sum solution using the EBP register (content of the ESP changes push/pop) standard way of parameter access EBP, ESP EAX, [EBP+4] storing the content of the EBP (can be used by other procedures) push EBP EBP, ESP

7 information in the stack (parameters, urn address, old EBP, local variable eventually) stack frame [1] stack after storing the EBP (a) before the procedure termination restoring the EBP (b) after execution of instruction (c) reing parameters from the stack modifying the ESP by er procedure (e.g. C-language) push number1 push number2 sum add ESP, 4 (if 2 variables, each of size 2B) ee procedure value (EIP SS:ESP, ESP ESP value) freeing the stack comparing strategies procedures with fixed number of parameters (preferred the second method) procedures with variable number of parameters (first method required)

8 Storing the state of a procedure e.g. ed procedure manipulates with register used by er one register contents usually stored by ed procedure reasons (changes in ed procedure, multiple s longer code) which registers to store? generally: used by er, modified by ed procedure push vs. pushad speed (5x, effective when > 5 registers stored) urn value in register (gcc EAX), popa overwrites this value modification of parameter access (offset) pusha EBP, ESP Instructions ENTER and LEAVE enter stack frame allocation when entering the procedure enter bytes, level bytes number of bytes for local variables, level procedure nesting level (usually 0) leave reing the stack frame (without parameters) proc-name: enter XX, 0 leave enter push EBP ESP, EBP EBP, ESP pop EBP sub ESP, XX XX,0... procedure body... leave YY

9 Example: parameter passing, stack method %include "asm_io.inc" segment.data prompt_msg1 db "Enter first number: ",0 prompt_msg2 db "Enter second number: ",0 sum_msg db "The sum is: ",0 segment.text global _asm_main _asm_main: enter 0,0 pusha print_nl EAX, sum_msg print_string sum print_int print_nl popa EAX, 0 leave push push EAX, prompt_msg1 print_string read_int EAX print_nl EAX, prompt_msg2 print_string read_int EAX sum: enter 0,0 ; save EBP EAX,[EBP+12] add EAX,[EBP+8] leave ; restore EBP 8 ; params

10 Procedures with variable number of parameters common appearance in C language (scanf, printf) ed procedure needs an information about the number of parameters last parameter stored onto stack gives the number of parameters parameters from the stack reed by er

11 Local variables appear at procedure activation, disappear at procedure termination (dynamic) allocation in data segment (DS) unwanted: in DS - static allocation (accessible also when the procedure is not active) doesn't work correctly with recursive procedures (s themselves directly/indirectly) and thus allocation in the stack [1] Example: access to stack frame EBP (frame pointer) parameters (a, b EBP+12, EBP+8) local variables (temp, N EBP-4, EBP-8) optional program clarification: %define a dword [EBP+12] %define temp dword [EBP-4] Usage: EBX, a instead of: EBX, [EBP+12] temp, EAX [EBP-4], EAX

12 Multi-module programs real applications (often a number of procedures) splitting source code to smaller parts (modules) advantages changes in module, compilation (only) of given module easier, safer modification (smaller files) separate module compilation interface specification required NASM directives GLOBAL and EXTERN Directive GLOBAL makes label accessible to other modules (procedure names, variables, ) global Directive EXTERN label1, label2, global error_txt, sum, mylabel.data error_txt db 'Out of range!',0 sum dw 0....CODE... mylabel:... informs the assembler, that label is not defined in actual module (defined in other) extern label1, label2, (label1, label2 made accessible in other module using GLOBAL) Study literature: [1] Dandamudi,S.,P.: Introduction to Assembly Language Programming, Springer Science+Business Media, Inc., [2] Carter, A., P.: PC Assembly Language, 2006, [3] NASM - The Netwide Assembler, The NASM Development Team, 2007,