University of Tehran 1 Microprocessor System Design Instructions (1)

University of Tehran 1

Microprocessor System Design

Instructions (1)


Outline

• Data transfer operations

• Arithmetic operations

• Logic operation

• Control operations

• String operations


Data Transfer Instructions• Very Common Instruction: mov desti, source

• Allowed Operands

Destination SourceMemory AccumulatorAccumulator MemoryRegister RegisterRegister MemoryMemory RegisterRegister ImmediateMemory ImmediateSeg. Reg. RegisterSeg. Reg. MemoryRegister Seg. Reg.Memory Seg. Reg.


Arithmetic Instruction Summary

add ax, bx ;axax+bx and set flagsadc ax, bx ;axax+bx+CF(lsb) and set flagsinc ax ;axax+1 and set flagsdaa ;Decimal (BCD) Adjust after Additionsub ax, bx ;axax-bx and set flagssbb ax, bx ;ax(ax-CF)-bx and set flagsdec ax ;axax-1neg ax ;ax(-1)*(ax) -- 2’s Complementcmp ax, bx ;Flags are set according to ax-bxdas ;Decimal (BCD) Adjust after Sub.mul cx ;dx:ax ax * cx (unsigned)imul cx ;dx:ax ax * cx (2’s complement)div cl ;alax/cl Quot. AND ahax/cl Rem.idiv cx ;ax(dx:ax)/cx Quot. AND dx Rem.


Increment Examples

inc bl ;blbl+1 and set flags

inc BYTE PTR [bx] ;Byte at ds:(bx)ds:(bx)+1

New MASM Directive: BYTE POINTER

00ffh 0000h

inc [bx] ;Word at ds:(bx)ds:(bx)+1

00ffh 0100h

inc [DATA1] ;ds:(DATA1)ds:(DATA1)+1


Add with Carry

add ax, cx ;axax+cx and flags setadc bx, dx ;bxbx+dx+CF(lsb) and flags set

33-bit Sum Present in CF:bx:ax

BX

DX

AX

CX

1 1

0 1

CF=1

BX AXCF


Decimal Adjust after Addition

• For BCD Arithmetic

• “Corrects” Result

0110 6 +0111 7 1101 13should be 0001 0011 (1101 is illegal BCD)

•daa Uses Implicit Operand, al Register

•Follows add, adc to “Adjust”


Decimal Adjust after Addition Example

mov dx, 1234h ;dx1234 BCDmov bx, 3099h ;bx3099 BCDmov al, bl ;al99 BCDadd al, dl ;alcdh illegal BCD, need 34+99=133daa ;al33h (33 BCD) and CF=1mov cl, al ;cl33 BCDmov al, bh ;al30 BCDadc al, dh ;al30h+12h+1=43hdaa ;al43h (43 BCD) not illegal BCD this timemov ch, al ;cx=4333h BCD for 1234+3099


Subtraction Instruction Types

sub ax, bx ;axax-bx and set flags

sbb ax, bx ;ax(ax-CF)-bx and set flags

dec ax ;axax-1

neg ax ;ax(-1)*(ax) - 2’s Complement

cmp ax, bx ;Flag is set according to ax-bx

das ;Decimal (BCD) Adjust after Subtraction


Allowable Operands for add, sub

Gen Reg

Gen Reg

Mem Loc

Immediate

+-

Gen Reg

Mem Loc

Immediate

+-

Destination Source


Subtract with Borrow, sbb

sub ax, di ;axax-di and CF gets borrow bitsbb bx, si ;bx(bx-CF(lsb))-si and flags set

32-bit Difference Present in bx:axCF Indicates If Difference is Negative

BX

SI

AX

DI

CF

BX AXCF


Multiplication

• 8086/8088 One of First to Include mul/div Instruction

• Allowable Operands: Bytes, Words, DoubleWords

•Allowable Results: Words, DoubleWords, QuadWords

•OF, CF Give Useful Information

•AF, PF, ZF, SF Change but Contents Unpredictable

•Multiplicand Always in al, ax

•mul - Unsigned Mnemonic

•imul - Signed Mnemonic


Multiply Instructions

• Product can be Twice the Size2 3 = 6 (same size)

2 8 = 16 (double size, EXT)

•OF=CF=0 means product is same size as result

•OF=CF=1 means EXT product size

•AF, PF, ZF, SF Contents Unpredictable

mul bl ;axal*bl, Unsignedmul bx ;dx:axbx*ax, Unsignedimul bl ;axal*bl, Signedimul bx ;dx:axbx*ax, Signed


Division

• 8, 16 bit Operands

• No Immediate Addressing Mode

• No Flag Bits Change Predictably

• Operands: Divisor is Programmer Specified

• Dividend is Implied

• Quotient, Remainder Implied

Size Dividend Quotient Remainder 8 bits ax al ah

16 bits dx:ax ax dx


Division Instruction Examples

• idiv Signed and div Unsigned

dividend / divisor = quotient, rmdr

div cx ;dx:ax is divided by value in cx;unsigned quotient is placed in ax;positive remainder is placed in dx


Logical Instructions


Logic Instruction TypesBITWISE LOGICAL

not ax ;1’s Complement-Logical Invertand ax, bx ;Bitwise logical and operationor ax, bx ;Bitwise logical inclusive-or operationxor ax, bx ;Bitwise logical exclusive-or operationtest ax, fffh ;Bitwise and but result discarded

SHIFTshl ax, 4 ;Logical shift leftsal ax, 3 ;Arithmetic shift leftshr ax, 4 ;Logical shift rightsar ax, 3 ;Arithmetic shift right

ROTATErol bx, 3 ;Rotate leftror cx, 4 ;Rotate rightrcl ax, 1 ;Rotate left through carryRcr dx, 6 ;Rotate right through carry


Bit Level Logic

and, or, xor, not, test,

• Affect Status Flags as Follows:1) Always Clears CF and OF2) SF, ZF, AF, PF Change to Reflect Result

• Common Usage:

and ax, ax ;clear CF and OF

xor ax, ax ;clear ax=CF=OF=PF=AF=SF=0 and ZF=1


Masking OperationsXXXX XXXX (unknown word)

(AND) 0000 1111 (mask word)0000 XXXX (result)

What if we wanted 1111 XXXX instead?

EXAMPLE: Convert ASCII to BCD

;First convert to BCD - change 3235h into 0025hmov bx, 3235h ;bx ‘25’and bx, 0f0fh ;bx0205hmov dx, bx ;dx0205hshl bh, 4 ;bh20hor bl, bh ; bl = bh or bl = 20 or 05 = 25hxor bh, bh ;zero out bh, so bx = 0025 (BCD value)


Bit Test Instruction, test

• Same as and But Result is Discarded

• Only Affects Flags (like cmp)

• Use test for Single Bit and cmp for Byte, Word

• ZF=1 if Tested Bit=0 and ZF=0 if Tested Bit=1

test al, 1 ;XXXX XXXX (AND) 0000 0001test al, 128 ;XXXX XXXX (AND) 1000 0000


Shiftsshl - Logical Shift Left

REGCF 0

REG CF0

REGCF 0

REG CF

shr - Logical Shift Right

sal - Arithmetic Shift Left (same as logical)

sar - Arithmetic Shift Right (sign bit is preserved)

MSB


Simple Arithmetic Using Shifts

;Compute (-3)*VALUE Using Only Shifts and Adds

mov ax, VALUE ;ax Word from memory with label VALUEmov bx, ax ;bx Word from memory with label VALUEshl ax, 2 ;ax 4*VALUEadd ax, bx ;ax 5*VALUEshl bx, 3 ;bx 8*VALUEsub ax, bx ;ax (-3)*VALUE


Rotatesrol - Rotate Left

REGCF

rcl - Rotate Through Carry Left

ror - Rotate Right

rcr - Rotate Through Carry Right

REGCF

REGCF

REGCF


Example Using Rotates

;Multiply a 48-bit value in dx:bx:ax by 2

shl ax, 1 ;ax 2*axrcl bx, 1 ;bx 2*bx + CF(lsb)rcl dx, 1 ;dx 2*dx + CF(lsb)

;End result is dx:bx:ax 2*(dx:bx:ax)

• Operand for rotates and shifts can be either:

1) Immediate value

2) Quantity in cl


Program Control Instructions


Program Control Instructions

• Generally modify CS:IP • Causes modification in execution sequence (of instructions)• When such a program flow change occurs:

a) Instructions in the BIU inst. queue become invalid

b) BIU directly fetches CS:IP instruction from memory

c) While EU executes new instruction, BIU refills inst. Queue

• Classificationa) Jumps - Unconditional control transfers

b) Branches - Conditional control transfer

c) Interrupts - Unconditional control transfers (asynchronous)

d) Iteration – More complex type of branch


Control Instruction Summary

UNCONDITIONALjmp LABEL ;next instruction executed has LABELcall LABEL ;next instruction executed has LABELret ;next instruction executed is after the callhlt ;nothing executed until RESET signal

ITERATIONloop LABEL ;cx cx - 1, jump to LABEL if cx > 0loope/loopz LABEL ;same as loop but ZF=1 also requiredloopne/loopnz LABEL ;same as loop but ZF=0 also required

INTERRUPTSint <immed8> ;Invoke the int. handler specified by immed8 iret ;Return from interrupt handler


Simplest Control Instruction, jmp

jmp LABEL ;LABEL is offset address of instruction ;in the code segment

3 Forms of jmp

SHORT - 2 bytes, allows jump to ±127 locations from current address

NEAR - 3 bytes, allows jump to ±32K locations from current address

FAR - 5 bytes anywhere in memory

EB disp

E9 disphi displo

EA IP lo IP hi CS lo CS hi


Example with Short Jump

;Causes bx to count by 1 from 0 to 65535 to 0 to 65535 to …

xor bx, bx ;Clear bx and initialize status flagsstart: mov ax, 1 ;ax 1

add ax, bx ;ax ax+bxjmp next ;add a displacement to IP

;(+2 from xor to mov)xor bx, bx ;Clear bx and initialize flagsxor ax, ax ;Clear ax and initialize flags

next: mov bx, ax ;bx axjmp start ;add a displacement to IP

;(a negative value - 2’s comp.)


Indirect Jump

jmp ax ;ip ax

• Address of target is in register

• Does NOT add disp to IP - Transfer REG contents to IP


Conditional Control Instruction SummarySimple Flag Branches

CONDITIONAL

jc LABEL ;jump on carry (CF=1)jnc LABEL ;jump on no carry (CF=0)je/jz LABEL ;jump if ZF=1 - jump if equal/zerojne/jnz LABEL ;jump if ZF=0 - jump not equal/jump if zerojo LABEL ;jump if OF=1 - jump on overflowjno LABEL ;jump if OF=0 - jump if no overflowjs LABEL ;jump on sign flag set (SF=1)jns LABEL ;jump if no sign flag (SF=0)jp/jpe LABEL ;jump if PF=1 - jump on parity/parity evenjnp/jpo LABEL ;jump if PF=0 - jump on no parity/parity odd

Jump based on single flag


Conditional Control Instruction SummaryBranches for unsigned comparisons

Jump is based on flags used for unsigned number comparison (based on C, Z flag)

CONDITIONALja/jnbe LABEL ;jump if CF=ZF=0 - jump above-jump not below/equaljae/jnb LABEL ;jump if CF=0 - jump above/equal-jump not belowjb/jnae LABEL ;jump if CF=1 - jump below-jump not above/equaljbe/jna LABEL ;jump if CF=1 or ZF=1 - jump equal - jump zero

Typical use: cmp al,bljb there ; jump if al is ‘below’ bl

; unsigned comparison


Conditional Control Instruction SummaryBranches for signed comparisons

Typical use: cmp al,bljl there ; jump if al is less than bl

; signed comparison

Jump is based on flags used for signed number comparison (based on Z, S, V flags)

CONDITIONAL

jg/jnle LABEL ;jump if ZF=0 and (SF=OF) - jump greater/not less ; nor equal

jge/jnl LABEL ;jump if SF=OF - jump greater-equal/not less thanjl/jnge LABEL ;jump if SF OF - jump less than/not greater nor

; equal jle/jng LABEL ;jump if ZF=1 or SF OF - jump less or equal/not

; greater than


Iteration Instruction, loop

• Combination of decrement cx and conditional Jump

• Decrements cx and if cx0 jumps to LABEL

Example:

ADDS PROC NEARmov cx, 100 ;cx 64h - number of words to addmov si, OFFSET BLOCK1 ;si offset of BLOCK1 (in ds)mov di, OFFSET BLOCK2 ;di offset of BLOCK2 (in es)cld ;Auto-increment si and di, DF=0

AGAIN: mov bx, di ;bx di, save offset of BLOCK2lodsw ;ax ds:[si], sisi+2, didi+2add ax, [bx] ;ax ax + ds:[bx]mov di, bx ;di bx, restore di with

; offset in BLOCK2stosw ;es:[di] ax, sisi+2, didi+2loop AGAIN ;cx cx - 1, if cx0 jump to AGAINret ;ip ss:[sp]

ADDS ENDP


Procedures

• Group of instructions that perform single task– (can be used as) a SUBROUTINE

call - invokes subroutine - pushes ipret - returns from subroutine - pops ip

• Uses MASM directives: PROC and ENDP

• Must specify

NEAR - intrasegmentFAR - intersegment

• Difference is op-code of ret

NEAR - c3h - pops IPFAR - cbh - pops CS, pops IP


call Instruction

• Differs from jmp since return address on stack

NEAR call: 3 bytes - 1 opcode and 2 for IP FAR call: 5 bytes - 1 opcode, 2 for IP and 2 for CS

• call with operand - can use 16-bit offset in any register except segment registers

call bx ;pushes ip then jumps to cs:[bx]


String Transfer Instructions

• String Forms:

movsb ;move string byte by bytemovsw ;move string word by word

EXAMPLE:movsb ;Copies 8 bits at DS:SI to ES:DI


Other String Instructions

lodsb ;loads al with contents of ds:si;Inc/Dec si by 1 depending on DF

lodsw ;loads ax with ds:si ;Inc/Dec si by 2 depending on DF

stosb ;loads es:di with contents of al;Inc/Dec di by 1 depending on DF

stosw ;loads es:di with contents of ax;Inc/Dec di by 2 depending on DF


String Scan Instruction, scas•scasb, scasw,

• Compares al, ax with memory data

• Does an integer subtraction - result not saved

• Generally used with a REPEAT prefix

•DF controls auto-increment/decrement

•Example:

mov di, OFFSET BLOCK ;di address of memory location BLOCKcld ;DF 0, auto-increment modemov cx, 100 ;cx 64h, initialize counter to 100xor al, al ;clear alrepne scasb ;test for 00h in location es:di

;if es:di not equal to 00h then; cx cx - 1, di di + 1, repeat;else if cx = 00h; do not repeat test;else if es:di equals 00h; ZF = 1, do not repeat test


Compare String Instruction, cmps

•cmpsb, cmpsw,

• Compares 2 sections of memory

• Does an integer subtraction - result not saved

• Generally used with a REPEAT prefix

•si, di auto-increment/decrement depending on DF

•Example: Test two strings for equivalence

;Assume that ds and es are already set-up (NOTE:ds can equal es)lea si, LINE ;si gets offset of location labeled LINElea di, TABLE ;di gets offset of location labeled TABLEcld ;DF=0, auto-increment modemoc cx, 10 ;initialize counter register to 10repe cmpsb ;while ds:si=es:di decrement cx and incr. si, di

;if cx=0 stop testing;after complete, if cx not equal 0, then;strings do not match

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University of Tehran 1 Microprocessor System Design Instructions (1)