1. CE302 Outline Multiplication Division Program Segment Prefix Command Line Parameters

2. CE302 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

3. CE302 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 WORD PTR [SI] ; AX * [word contents of memory location ; addressed by SI] (signed product in DX--AX)

4. CE302 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)

5. CE302 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

6. CE302 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)

7. CE302 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

8. CE302 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

9. CE302 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

10. CE302 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

11. CE302 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 ones 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

12. CE302 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)

13. CE302 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

14. CE302 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:

15. CE302 Program Segment Prefix (PSP) When a program is loaded into memory for execution, DOS first builds up a program segment prefix immediately before the program is loaded into memory. This PSP contains lots of information, some of it useful, most of it obsolete. Understanding the layout of the PSP is essential for programmers designing assembly language programs. The PSP is 256 bytes long

16. CE302 Program Segment Prefix (PSP) Offset 0 2 4 5 0Ah 0Eh 12h 16h 2Ch 2Eh 50h 53h 5Ch 6Ch 80h 81h Length 2 1 5 4 22 2 34 3 9 16 20 1 127 Description An INT 20h instruction is stored here Program ending address Unused, reserved by DOS Call to DOS function dispatcher Address of program termination code Address of break handler routine Address of critical error handler routine Reserved for use by DOS Segment address of environment area Reserved by DOS INT 21h, RETF instructions Reserved by DOS Default FCB #1 Default FCB #2 Length of command line string Command line string

17. CE302 PSP – Program Ending Address Field number two contains a value which points to the last memory address allocated to your program. By subtracting the address of the PSP from this value, you can determine the amount of memory allocated to your program

18. CE302 PSP – Environment Area Address This field contains the segment address of the environment storage area The environment strings always begin from an offset of zero from the above segment address This area of memory consists of a sequence of zero- terminated strings using the format: string1 0 string2 0 string3 0 0 Strings are usually placed in the environment area using DOS commands like PATH or SET Generally an environment string takes the form name = parameters

19. CE302 PSP – Environment Area Address For example the statement set ipath=c:\assembly\include copies the string “ipath=c:\assembly\include” into the environment string storage area. Many programs scan the env storage area for paths and other information. Your programs can take advantage of this too.

20. CE302 PSP – Command Line String Many programs allow you to append parameters after the executable name: e.g. Notepad mydoc.txt This command line string is stored in the PSP Location 80h of the PSP stores the length of the CLS Locations 81h through FFh contain the string itself You can use CLS in your ASM programs just like you would use argc, argv to access command line parameters in C/C++

21. CE302 PSP – Command Line String For example, consider MYPGM parameter1, parameter2 The CLS for this will be 23, “ parameter1, parameter2”, 0Dh Notice that the carriage return character is not figured into the length Please read Section 13.3.12 of the online text for an in depth discussion of this topic with parsing examples

22. CE302 Accessing the PSP Although the PSP is loaded into memory immediately before your program, that doesn’t necessarily mean that it appears 100h bytes before your code Your data segments may have been loaded into memory before your code segments, thereby invalidating this method of locating the PSP The segment address of the PSP is passed to your program in the DS register Use the following code to extract the PSP segment address

23. CE302 Accessing the PSP Push ds;Save PSP value in the stack Mov ax, seg DSEG;Point DS and ES to our data segment Mov ds, ax Mov es, ax Pop PSP;Store PSP segment address into “PSP variable

24. CE302 Accessing the PSP In DOS 5.0 and later, you can make a DOS call to obtain the PSP address Load AH with 51h and execute an int 21h instruction DOS will return the segment address of the current PSP in the bx register