1 Lecture 5: Procedures Assembly Language for Intel-Based Computers, 4th edition Kip R. Irvine

Outline  Stack OperationsStack Operations  Defining and Using ProceduresDefining and Using Procedures

Stack Operations n Why Stacks?  Recall data structures class.  Recall how procedure (functions) used.  Procedure calling is a stack operation.  We use stack to keep track of return addresses.  Parameters and local variables are also put on the stack when calling some subroutines.

Stack Operations n Concepts  A stack is a LIFO (last-in, first-out) structure  The runtime stack is a memory that is managed directly by the CPU, using two registers: SS and ESP  SS holds a segment descriptor and is not modified by user program  ESP holds a 32-bit offset into some location on the stack  An Intel stack grows downward from high memory to low memory

 Format: PUSH r/m16/m32/imm16/imm32 Stack Operations n PUSH Operation  Immediate values are 32-bit in protected mode and 16-bit in real mode  A 16-bit/32-bit operand causes ESP to be decremented by 2 and 4, respectively. Push AX Push BX Offset Stack AB AX BX AB ESP XX YY ESP ESP AB 01

 Format: POP r/m16/m32 Stack Operations n POP Operation POP BX POP AX Offset Stack AB AX BX ? 0000? XX YY ESP AB 01 ESP 01AB 0024 ESP  The area of the stack above ESP is logically empty, and will be overwritten

Stack Operations n Additional Stack Operations  PUSHF and POPF Push and pops the Flag register. There are no operands  PUSHAD and POPAD Pushes registers on the stack in this order: EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI and pops them in reverse order  PUSHA and POPA The same except they work with 16-bit registers

Outline  Stack OperationsStack Operations  Defining and Using ProceduresDefining and Using Procedures

Procedures n Concepts  Procedure: blocks of code that are called and must be returned from  A procedure begins with itsname proc and terminate with itsname endp  To end a procedure other that the program startup procedure (main), use ret instruction  Use call itsname to call the procedure.  It is a highly desirable to preserve registers when writing a procedure. Save at beginning and restore before returning

Procedures n CALL and RET Instructions  CALL: directs the processor to begin execution at a new memory location  Pushes the return address on the stack  Copies the called procedure address into the EIP.  RET: brings the processor back to the point in the program where the procedure was called  Popes the return address from the stack into the EIP.

??? Procedures n Examples main PROC Call MyProc MOV eax, ebx … MyProc PROC MOV eax, edx … ret MyProc Endp ESP EIP ??? ESP EIP

Procedures n Nested Procedures ESP (return to sub2) (return to sub1) (return to main) Low high

Procedures n Local and Global Labels  Global labels are followed by two colons, making them visible to the whole program  By default, a code label (followed by a single colon) has local scope, making it visible only to statements inside its enclosing procedure main PROC Sub PROC JMP L2 L2: L1:: JMP L1 main endp ret … ?

Procedures n Passing Parameter  In registers - Fastest  In global variables - Hard to reuse, poor programming practice  On the stack - Used by high level languages

Procedures n Examples  Calculating the sum of an integer array  It is better to pass the offset of an array to the procedure than to include references to specific variable names inside the procedure

Save and restore registers that are modified by a procedure The return register should not be pushed and popped

Procedures n Design using Procedures  Divide the specification into tasks  Understand the specification of the program  Design each task as a sub-procedure  Design the main procedure (the program start up procedure) to call all sub- procedures

Linking to External Library n Link Library  A file containing procedures that have been assembled into machine code  Linker command  In your program, these procedure could be included and called  The assembler would leave the target address of the Call instruction blank, which will be filled by the linker  link32 %1.obj libraryName.lib

Outline  Stack OperationsStack Operations  Defining and Using ProceduresDefining and Using Procedures