Outline Multiplication Division Program Segment Prefix Command Line

Outline • • Multiplication Division Program Segment Prefix Command Line Parameters CE 302

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: (216 - 1) * (216 - 1) = (232 - 2 * 216 + 1 < (232 - 1) signed: (-215) * (-215) = 230 < (231 - 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 CE 302

Multiplication (cont. ) • Two different instructions for multiplication – MUL – IMUL Multiply unsigned Integer 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) CE 302

Multiplication (16 bit) The use of the AX (and DX) registers is implied!!!!! Multiplicand Multiplier AX (16 -bit register, 16 -bit memory variable) DX, AX = PRODUCT (High word in DX : Low word in AX) CE 302

Multiplication • 8 -bit multiplication Multiplicand Multiplier AX AL (8 -bit register, 8 -bit memory variable) PRODUCT • 32 -bit multiplication Multiplicand Multiplier EAX (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 CE 302

Binary Multiplication • Long Multiplication is done through shifts and additions 0 1 1 0 0 0 1 0 (98) x 0 0 1 0 1 (37) ------------01100010 - 0 1 1 0 0 0 1 0 - - - (3626) • 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 CE 302

Division • X / Y = Q; R X Dividend Y Divisor Q Quotient R Remainder Examples (Signed Integers) Note: Remainder has the same sign as X (Dividend) CE 302 X/Y Q R 9/4 -9 / 4 9 / -4 -9 / -4 2 -2 -2 2 1 -1

Division (cont. ) • Two different instructions for division – DIV – IDIV Division unsigned Integer 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 CE 302

Division (32 -bit/16 -bit) The use of the AX (and DX) registers is implied!!!!! Dividend Divisor Quotient Remainder DX, AX (high word in DX, low word in AX) (16 -bit register, 16 -bit memory variable) AX DX CE 302

Division (cont. ) • 16 -bit/8 -bit Dividend Divisor Quotient Remainder AX (8 -bit register, 8 -bit memory variable) AL AH • 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) Quotient EAX Remainder EDX Available on 80386 and above CE 302

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 snnn nnnn (before) AX = ssss snnn nnnn (after) – CWD (convert word to double) DX: AX = xxxx snnn nnnn (before) DX: AX = ssss snnn nnnn (after) – CWDE (convert double to double-word extended) - 80386 and above CE 302

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) CE 302

Division (Example) Division of the byte contents of memory NUMB by the contents of NUMB 1 Unsigned MOV DIV MOV AL, NUMB AH, 0 NUMB 1 ANSQ, AL ANSR, AH Signed ; get NUMB ; zero extend MOV CBW IDIV ; save quotient MOV ; save remainder MOV CE 302 AL, NUMB 1 ANSQ, AL ANSR, AH ; get NUMB ; signed-extend ; save quotient ; save remainder

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 DIV ADD CMP JB INC BL AH, AH AH, BL NEXT AL NEXT: CE 302 ; double remainder ; test for rounding

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 CE 302

Program Segment Prefix (PSP) Offset 0 2 4 5 0 Ah 0 Eh 12 h 16 h 2 Ch 2 Eh 50 h 53 h 5 Ch 6 Ch 80 h 81 h Length 2 2 1 5 4 4 4 22 2 34 3 9 16 20 1 127 Description An INT 20 h 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 21 h, RETF instructions Reserved by DOS Default FCB #1 Default FCB #2 Length of command line string CE 302

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 CE 302

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 zeroterminated strings using the format: string 1 0 string 2 0 string 3 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 CE 302

PSP – Environment Area Address • For example the statement set ipath=c: assemblyinclude copies the string “ipath=c: assemblyinclude” 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. CE 302

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 80 h of the PSP stores the length of the CLS • Locations 81 h 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++ CE 302

PSP – Command Line String • For example, consider MYPGM parameter 1, parameter 2 • The CLS for this will be 23, “ parameter 1, parameter 2”, 0 Dh • 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 CE 302

Accessing the PSP • Although the PSP is loaded into memory immediately before your program, that doesn’t necessarily mean that it appears 100 h 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 CE 302

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 CE 302

Accessing the PSP • In DOS 5. 0 and later, you can make a DOS call to obtain the PSP address • Load AH with 51 h and execute an int 21 h instruction • DOS will return the segment address of the current PSP in the bx register CE 302