1. ECE291 Computer Engineering II Lecture 6 & Lecture 7 Dr. Zbigniew Kalbarczyk University of Illinois at Urbana- Champaign

2. Z. KalbarczykECE291 Outline Program organization MASM directives Multiplication Division

3. Z. KalbarczykECE291 Program Organization Create block structure and/or pseudocode on paper to get a clear concept of program control flow and data structures Break the total program into logical procedures/macros Use jumps, loops, etc. where appropriate Use descriptive names for variables –noun_type for types –nouns for variables –verbs for procedures/functions

4. Z. KalbarczykECE291 Debugging Hints Good program organization helps Programs do not work the first time Strategy to find problems –Use DEBUG breakpoints to check program progress –Use COMMENT to temporarily remove sections of code –"print" statements announce milestones in program Test values/cases –Try forcing registers/variables to test output of a procedure –Use "print" statements to display critical data Double-check your own logic (Did you miss a special case?) Try a different algorithm, if all else fails...

5. Z. KalbarczykECE291 MASM Directives General –TITLEmy_program.asm Includesdrive:\path\filename Definitions –DB define byte (8bits) –DWdefine word (16 bits) –DDdefine doubleword (32 bits) –EQUnames a constant Labels

6. Z. KalbarczykECE291 MASM Directives Macros –Instead of using procedures, which require both stack and time resources, macros are fast and flexible –Advantages: speed; no call instruction readability - easier to understand program function –Drawbacks - space using the MACRO multiple times duplicates the code tricky to debug (in particular when you have nested MACROs) Procedures –“name” PROC NEAR/FAR –ENDP

7. Z. KalbarczykECE291 MASM Directives (cont.) References to procedures –EXTERN“name” NEAR/FAR/BYTE/WORD –PUBLIC “ name” Segment definition –SEGMENT “name” PUBLIC/STACK –ENDS Segment register “Hooks” –ASSUME CS:CSEG; DES:CSEG; SS:STACK

8. Z. KalbarczykECE291 Example Program Structure TITLE ECE291:MPXXX COMMENT * In this MP you will develop program which take input from the keyboard………… * ;====== Constants================================================= ;ASCII values for common characters CR EQU 13 LF EQU 10 ESCKEY EQU 27 ;====== Externals ================================================= ; -- LIB291 Routines extrn dspmsg:near, dspout:near, kbdin:near extrn rsave:near, rrest:near, binasc:near

9. Z. KalbarczykECE291 Example Program Structure (cont.) ;==== LIBMPXXX Routines (Your code will replace calls to these functions) extrn LibKbdHandler:near extrn LibMouseHandler:near extrn LibDisplayResult:near extrn MPXXXXIT:near ;====== Stack ===================================================== stkseg segment stack ; *** STACK SEGMENT *** db 64 dup ('STACK ') ; 64*8 = 512 Bytes of Stack stkseg ends ;====== Begin Code/Data ============================================ cseg segment public 'CODE' ; *** CODE SEGMENT *** assume cs:cseg, ds:cseg, ss:stkseg, es:nothing

10. Z. KalbarczykECE291 Example Program Structure (cont.) ;====== Variables ================================================= inputValid db 0 ; 0: InputBuffer is not ready ; 1: InputBuffer is ready ;-1: Esc key pressed operandsStr db 'Operands: ','$' OutputBuffer db 16 dup(?),'$' ; Contains formatted output ; (Should be terminated with '$') MAXBUFLENGTH EQU 24 InputBuffer db MAXBUFLENGTH dup(?),'$' ; Contains one line of ; user input include graphData.dat ; data PUBLIC OutputBuffer, inputValid, operandsStr PUBLIC graphData

11. Z. KalbarczykECE291 Example Program Structure (cont.) ;====== Procedures =========================================== KbdHandler PROC NEAR KbdHandler ENDP MouseHandler PROC NEAR MouseHandler ENDP DisplayResult PROC NEAR DisplayResult ENDP

12. Z. KalbarczykECE291 Example Program Structure (cont.) ;====== Main Procedure ======================================== MAIN PROC FAR MOV AX, CSEG ;Use common code and data segment MOV DS, AX MOV AX, 0B800h ;Use extra segment to access video screen MOV ES, AX CALL MPXXXXIT ; Exit to DOS MAIN ENDP CSEG ENDS END MAIN

13. Z. KalbarczykECE291 Multiplication The product after a multiplication is always a double-width product, e.g, –if we multiply two 16-bit numbers, they generate a 32-bit product –unsigned: (2 16 - 1) * (2 16 - 1) = (2 32 - 2 * 2 16 + 1 < (2 32 - 1) –signed: (-2 15 ) * (-2 15 ) = 2 30 < (2 31 - 1) –overflow cannot occur Modification of Flags –Most flags are undefined after multiplication –O and C flags clear to 0 if the result fit into half-size register –e.g., if the most significant 16 bits of the product are 0, both flags C and O clear to 0

14. Z. KalbarczykECE291 Multiplication (cont.) Two different instructions for multiplication –MULMultiply unsigned –IMULInteger Multiply (2’s complement) Multiplication is performed on bytes, words, or double words Which operation to perform depends on the size of the multiplier The multiplier can be any register or any memory location mul cx; AX * CX (unsigned result in DX--AX); imul BYTE PTR [si] ; AX * [word contents of memory location ; addressed by SI] (signed product in DX--AX)

15. Z. KalbarczykECE291 Multiplication (16 bit) The use of the AX (and DX) registers is implied!!!!! Multiplicand AX Multiplier (16-bit register, 16-bit memory variable) DX, AX = PRODUCT (High word in DX : Low word in AX)

16. Z. KalbarczykECE291 Multiplication (cont.) 8086/8088 microprocessors do not allow to perform immediate multiplication 80286, 80386, and 80486 allow the immediate multiplication by using a special version of the multiply instruction Immediate multiplication must be signed multiplication and contains three operands –16-bit destination register –register or memory location that contains 16-bit multiplicand –8-bit or 16-bit immediate data used as a multiplier mulcx, dx, 12h ;multiplies 12h * DX and leaves ;16-bit signed product in CX

17. Z. KalbarczykECE291 Multiplication 8-bit multiplication Multiplicand AL Multiplier (8-bit register, 8-bit memory variable) AX PRODUCT 32-bit multiplication Multiplicand EAX Multiplier (32-bit register, 32-bit memory variable) EDX, EAX PRODUCT (High word in EDX : Low word in EAX) –32-bit multiplication is available only on 80386 and above

18. Z. KalbarczykECE291 Binary Multiplication Long Multiplication is done through shifts and additions This works if both numbers are positive To multiply a negative numbers, the CPU will store the sign bits of the numbers, make both numbers positive, compute the result, then negate the result if necessary 0 1 1 0 0 0 1 0 (98) x 0 0 1 0 0 1 0 1 (37) ------------------------- 0 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 - - 0 1 1 0 0 0 1 0 - - - - - (3626)

19. Z. KalbarczykECE291 Division X / Y = Q; R X Dividend YDivisor QQuotient RRemainder Note: Remainder has the same sign as X (Dividend) Examples (Signed Integers) X / YQR 9 / 4 2 1 -9 / 4-2-1 9 / -4-2 1 -9 / -4 2 1

20. Z. KalbarczykECE291 Division (cont.) Two different instructions for division –DIVDivision unsigned –IDIVInteger Division (2’s complement) Division is performed on bytes, words, or double words Which operation to perform depends on the size of the divisor The dividend is always a double-width dividend that is divided by the operand (divisor) The divisor can be any register or any memory location

21. Z. KalbarczykECE291 Division (32-bit/16-bit) The use of the AX (and DX) registers is implied!!!!! Dividend DX, AX (high word in DX, low word in AX) Divisor(16-bit register, 16-bit memory variable) QuotientAX RemainderDX

22. Z. KalbarczykECE291 Division (cont.) 16-bit/8-bit Dividend AX Divisor(8-bit register, 8-bit memory variable) QuotientAL RemainderAH 64-bit/32-bit Dividend EDX, EAX (high double word in EDX, low double word in EAX) Divisor(32-bit register, 32-bit memory variable) QuotientEAX RemainderEDX Available on 80386 and above

23. Z. KalbarczykECE291 Division (cont.) Division of two equally sized words Prepare the dividend –Unsigned numbers: move zero into high order-word –Signed numbers: use signed extension ( implicitly uses AL, AX, DX registers ) to fill high-word with once or zeros –CBW (convert byte to word) AX = xxxx xxxx snnn nnnn (before) AX = ssss ssss snnn nnnn (after) –CWD (convert word to double) DX:AX = xxxx xxxx xxxx xxxx snnn nnnn nnnn nnnn (before) DX:AX = ssss ssss ssss ssss snnn nnnn nnnn nnnn (after) –CWDE (convert double to double-word extended) - 80386 and above

24. Z. KalbarczykECE291 Division (cont.) Flag settings –none of the flag bits change predictably for a division A division can result in two types of errors –divide by zero –divide overflow (a small number divides into a large number), e.g., 3000 / 2 AX = 3000; Devisor is 2 => 8 bit division is performed Quotient will be written to AL => but 1500 does not fit into AL consequently we have divide overflow in both cases microprocessor generates interrupt (interrupts are covered later in this course)

25. Z. KalbarczykECE291 Division (Example) Division of the byte contents of memory NUMB by the contents of NUMB1 Unsigned MOVAL, NUMB ;get NUMB MOVAH, 0 ;zero extend DIVNUMB1 MOVANSQ, AL ;save quotient MOVANSR, AH ;save remainder Signed MOVAL, NUMB ;get NUMB CBW ;signed-extend IDIVNUMB1 MOVANSQ, AL ;save quotient MOVANSR, AH ;save remainder

26. Z. KalbarczykECE291 Division (cont.) What do we do with remainder after division? –use the remainder to round the result –drop the remainder to truncate the result –if the division is unsigned, rounding requires that remainder is compared with half the divisor to decide whether to round up the quotient –e.g., sequence of instructions that divide AX by BL and round the result DIVBL ADDAH, AH;double remainder CMPAH, BL;test for rounding JBNEXT INCAL NEXT: