1 / 31

Today’s topics

Today’s topics. Parameter passing on the system stack Register indirect and base-indexed addressing modes “Random” numbers Intro to arrays. CALL and RET Instructions. The CALL instruction calls a procedure Pushes the offset of the next instruction onto the stack

deo
Download Presentation

Today’s topics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Today’s topics • Parameter passing on the system stack • Register indirect and base-indexed addressing modes • “Random” numbers • Intro to arrays

  2. CALL and RET Instructions • The CALL instruction calls a procedure • Pushes the offset of the next instruction onto the stack • copies the address of the called procedure into EIP • The RET instruction returns from a procedure • pops top of stack into EIP

  3. Nested procedure calls • Any procedure might call another procedure • Return addresses are “stacked” (LIFO) • ret instructions must follow the order on the stack • One very good reason not to jmp out of a procedure! • It is essential that the stack be properly aligned when the RET instruction is executed !!

  4. Parameters • Definitions: • Argument (actual parameter) is a value or reference passedto a procedure. • Parameter (formal parameter) is a value or reference received by a procedure

  5. Register vs. Stack Parameters • Register parameters require dedicating a register to each parameter. • Stack parameters make better use of system resources • Example: • two ways of calling Summation procedure. Method 1 (parameters in registers): pushad ;save registers mov ebx,low mov ecx,high call Summation popad ;restore registers Method 2 (parameters on stack): push low push high call Summation

  6. Register vs. Stack Parameters • Of course, methods of calling a procedure and passing parameters depend on the procedure implementation … and vice-versa. • some “setup” is involved (in the calling procedure) • some “bookkeeping” is involved (in the called procedure) • Parameters in registers require register management • Parameters on the system stack require stack management

  7. RET Instruction • Pops stack into the instruction pointer (EIP) • Transfers control to the target address. • Syntax: • RET • RETn • Optional operand n causes n to be added to the stack pointer after EIP is assigned a value. • Equivalent to popping the return address and n additional bytes off the stack

  8. Parameter Classifications • An input parameter is data passed by a calling program to a procedure. • The called procedure is not expected to modify the corresponding parameter variable, and even if it does, the modification is confined to the procedure itself. • An output parameter is created by passing the address of a variable when a procedure is called. • The “address of” a variable is the same thing as a “pointer to” or a “reference to” the variable. In MASM, we use OFFSET. • The procedure does not use any existing data from the variable, but it fills in new contents before it returns. • An input-output parameter is the address of a variable which contains input that will be both used and modified by the procedure. • The content is modified at the memory address passed by the calling procedure.

  9. Stack Frame • Also known as an activation record • Area of the stack used for a procedure's return address, passed parameters, saved registers, and local variables • Created by the following steps: • Calling program pushes arguments onto the stack and calls the procedure. • The called procedure pushes EBP onto the stack, and sets EBP to ESP.

  10. Addressing modes • Immediate Set register to a constant • Direct Set register to address of global • Register Use register as operand • Register indirect Access memory through address in a register • Indexed “array” element, using offset in register • Base-indexed Start address in one register; offset in another, add and access memory • Stack Memory area specified and maintained as a stack; stack pointer in register • Offset (branch) “goto” address; may be computed

  11. Register indirect mode • [reg] means “contents of memory at the address in reg ” • It’s OK to add a constant (named or literal) • Example: mov [edx+12], eax • We have used register indirect with esp to reference the value at the top of the system stack • NOTE: register indirect is a memory reference • There are no memory-memory instructions • E.G., mov [edx],[eax] is WRONG!

  12. Explicit Access to Stack Parameters • A procedure can explicitly access stack parameters using constant offsets from EBP. • Example: [ebp + 8] • EBP is often called the base pointer or frame pointer because it is (should be) set to the base address of the stack frame. • EBP does not change value during the procedure. • EBP must be restored to its original value when a procedure returns.

  13. Explicit Access to Stack Parameters • Remember that the return address is pushed onto the stack after the parameters are pushed. • Programmer is responsible for managing the stack.

  14. Stack Frame Example .data x DWORD 175 y DWORD 37 Z DWORD ? .code main PROC push x push y push OFFSET z call SumTwo ... Note: @ means “address of”

  15. Stack Frame Example SumTwo PROC push ebp mov ebp,esp mov eax,[ebp+16] ;175 in eax add eax,[ebp+12] ;175+37 = 212 in eax mov ebx,[ebp+8] ;@z in ebx mov [ebx],eax ;store 212 in z pop ebp ret 12 SumTwo ENDP Note: @ means “address of”

  16. Trouble-Shooting Tips • Save and restore registers when they are modified by a procedure. • Except a register that returns a function result • Do not pass an immediate value to a procedure that expects a reference parameter. • Dereferencing its address will likely cause a general-protection fault.

  17. Random Numbers • Irvine library has random integer generator • "pseudo-random" numbers • Randomize procedure • Initializes sequence based on system clock (random seed) • Call once at the beginning of the program • Without Randomize, program gets the same sequence every time it is executed.

  18. Limiting random values • RandomRange procedure • Accepts N > 0 in eax • Returns random integer in [0 .. N-1] in eax • To generate a random number in [lo .. hi]: • Find number of integers possible in [lo .. hi] : range = hi – lo +1 • Put range in eax, and call RandomRange • Result in eax is in [0 .. range -1] • Add lo to eax.

  19. RandomRange Example • Get a random integer in range [18 .. 31] mov eax,hi ;31 sub eax,lo ;31-18 = 13 inc eax ;14 call RandomRange ;eax in [0..13] add eax,lo ;eax in [18..31] • Questions on random numbers?

  20. Arrays in MASM • Declaration (in data segment) MAX_SIZE = 100 .data list DWORD MAX_SIZE DUP(?) • Defines an array of 100 uninitialized 32-bit integers • Array elements are in contiguous memory

  21. Arrays in MASM • Declaration MAX_SIZE = 100 .data list DWORD MAX_SIZE DUP(?) count DWORD 0 • What happens (in HLL) if we reference list[100] ? • Compile-time error • What happens in MASM if we go beyond the end of the array? • Not predictable

  22. Array address calculations • Array declaration defines a name for the first element only • HLLs reference it as list[0] • All other elements are accessed by calculating the actual address • General formula for array address calculation: • address of list[k] = list + (k * sizeof element) • Example: • address of 4th element (list [3]) is address of list + (3 * sizeof DWORD)

  23. Array references in MASM • Several methods • indexed • base-indexed • others

  24. Addressing modes • Immediate Set register to a constant • Direct Set register to address of global • Register Use register as operand • Register indirect Access memory through address in a register • Indexed “array” element, using offset in register • Base-indexed Start address in one register; offset in another, add and access memory • Stack Memory area specified and maintained as a stack; stack pointer in register • Offset (branch) “goto” address; may be computed

  25. Indexed addressing • Array element, using offset in register • Examples: mov edi,0 ;high-level notation mov list[edi],eax ;list[0] add edi,4 ;see note below mov list[edi],ebx ;list[1] • This means "add the value in [ ] to address of list " • Note: Add 4 because these array elements are DWORD • if BYTE, add 1 • if WORD, add 2 • if QWORD, add 8 • etc.

  26. Base-indexed addressing • Start address in one register; offset in another, then add the registers to access memory • Examples: mov edx,OFFSET list mov ecx,12 mov eax,[edx+ecx] mov ebx,4 mov eax,[edx+ebx] mov [edx+ecx],eax

  27. Processing Arrays in MASM • Example: Display an array of integers • Assumptions: • Already initialized • Number of elements is stored in count • Assume global declarations • Assume elements are DWORD (32-bit) • Many possible solutions

  28. Display: version 0.1 (register indirect) display PROC mov esi,OFFSET list ;@list mov ecx,count ;ecx is loop control more: mov eax,[esi] ;get current element call PrintDec call Crlf add esi,4 ;next element loop more endMore: ret display ENDP

  29. Display: version 0.2 (indexed) display PROC mov esi,0 ;esi is “index” mov ecx,count ;ecx is loop control more: mov eax,list[esi] ;get current elt. call PrintDec call Crlf add esi,4 ;next element loop more endMore: ret display ENDP

  30. Display: version 0.3 (base-indexed) display PROC mov ebx,OFFSET list ;@list mov ecx,count ;ecx is loop control mov edx,0 ;edx is element ptr more: mov eax,[ebx+edx] call WriteDec call Crlf add edx,4 loop more endMore: ret display ENDP

  31. Questions?

More Related