Lecture 5 More on assembly language programming The S-bit ...

<strong>Lecture</strong> 5 <strong>More</strong> on assembly languageprogrammingThe S-bit◆The basic branch instruction is:◆The loop program can be simplied to:◆◆◆label labelB label label ;; unconditionally branch to to label label…… ………… …………. …….Conditional branch instructions can be used to control loops:MOV MOV r0, r0, #10 #10 ;; intialize loop loop counted r0 r0loop loop ……. ……. ;; start start of of body body of of loop loop……. …….SUB SUB r0, r0, r0, r0, #1 #1 ;; decrement loop loop counterCMP CMP r0, r0, #0 #0 ;; is is it it zero zero yet? yet?BNE BNE loop loop ;; branch if if r0 r0 ≠ 0c 0cThe CMP instruction gives no results EXCEPT possibly changingconditional codes in CPSR.If r0=0, then Z bit is set (='1'), else Z bit is reset (='0')◆◆◆◆MOV MOV r0, r0, #10 #10 ;; intialize loop loop counted r0 r0loop loop ……. ……. ;; start start of of body body of of loop loop……. …….SUBS SUBS r0, r0, r0, r0, #1 #1 ;; decrement loop loop counterBNE BNE loop loop ;; branch branch if if r0 r0 ≠ 0c 0cSUBS instruction is the same as SUB except that the former canchange the cc in CPSR.After SUBS instruction, Z-bit is set or cleared depending on the resultof the subtraction.All data processing instructions such as ADD, ADC etc. can have Sadded to indicate that condition code should be updated.We have seen how to use a conditional branch instruction BNE inloops. There are in fact many more conditional branch instructions.pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 1pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 2Conditional Branch InstructionsConditional Execution◆◆Conditional execution applies not only to branches, but to all ARMinstructions.For example:CMP CMP r0, r0, #5 #5 ;; if if (r0 (r0!= != 5) 5) then thenBEQ BEQ BYPASSADD ADD r1, r1, r1, r1, r0 r0 ;; r1 r1 := := r1 r1 + r0 r0 -- r2 r2SUB SUB r1, r1, r1, r1, r2 r2BYPASS ….. …..◆Can be replaced by:BYPASSCMP CMP r0, r0, #5 #5 ;; if if (r0 (r0!=!= 5) 5) then thenADDNE r1, r1, r1, r1, r0 r0 ;; r1 r1 := := r1 r1 + r0 r0 -- r2 r2SUBNE r1, r1, r1, r1, r2 r2….. …..◆Note that BCC = BLO, BCS = BHS◆Here the ADDNE and SUBNE instructions are executed only ifZ='0', i.e. the CMP instruction gives non-zero result.pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 3pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 4

Conditional Execution - moreConditional Execution - Summary◆Here is another very clever use of this unique feature in ARMinstruction set. Do remember ALL instructions can be qualified bythe condition codes.;; if if (((a==b) (a==b) && && (c==d)) (c==d)) then then e := := e + 1; 1;CMP CMP r0, r0, r1 r1 ;; r0 r0 has has a, a, r1 r1 has has bCMPEQ r2, r2, r3 r3;; r2 r2 has has c, c, r3 r3 has has dADDEQ r4, r4, r4, r4, #1 #1 ;; e := := e+1 e+1◆◆◆Note how if the first comparison finds unequal operands, thesecond and third instructions are both skipped.Also the logical 'and' in the if clause if implemented by making thesecond comparison conditional.Conditional execution is only efficient if the conditional sequence isthree instructions or fewer. If the conditional sequence is longer,use a proper loop.pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 5pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 6Shifted Register OperandsARM shift operations - LSL and LSR◆ARM has another very clever feature. In any data processinginstructions, the second register operand can have a shiftoperation applied to it. For example:◆Here are all the six possible ARM shift operations you can use:ADD ADD r3, r3, r2, r2, r1, r1, LSL LSL #3 #3 ;; r3 r3 := := r2 r2 + 8 x r1 r1◆◆◆◆Herer LSL means 'logical shift left by the specified number of bits.Note that this is still a single ARM instruction, executed in a singleclock cyle.In most processors, this is a separate instructions, while ARMintegrates this shifting into the ALU.It is also possible to use a register value to specify the number ofbits the second operand should be shifted by:ADD ADD r5, r5, r5, r5, r3, r3, LSL LSL r2 r2 ;; r5 r5 := := r5 r5 + r3 r3 x 2**r1 2**r1◆◆LSL: logical shift left by 0 to 31 places; fill the vacated bits at theleast significant end of the word with zeros.LSR: logical shift right by 0 to 32 places; fill the vacated bits at themost significant end of the word with zeros.pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 7pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 8

ARM shift operations - ASL and ASRARM shift operations - ROR and RRX◆◆ASL: arithmetic shift left; this is the same as LSLASR: arithmetic shift right by 0 to 32 places; fill the vacated bits atthe most significant end of the word with zeros if the sourceoperand was positive, and with ones it is negative. That is, signextend while shifting right.◆◆ROR: rotate right by 0 to 32 places; the bits which fall off the leastsignificant end are used to fill the vacated bits at the mostsignificant end of the word.RRX: rotate right extended by 1 place; the vacated bit (bit 31) isfilled with the old value of the C flag and the operand is shifted oneplace to the right. This is effectively a 33 bit rotate using theregister and the C flag.pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 9pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 10A simple assembly language program- Hello world!◆We will now consider two simple assembly language programs.The first outputs "Hello World!" on the console window:AREA AREA helloW, helloW, CODE, CODE, READONLY READONLY ;; declare declare code code area areaSWI_WriteC SWI_WriteC EQU EQU &0 &0 ;; output output character character in in r0 r0SWI_Exit SWI_Exit EQU EQU &11 &11 ;; finish finish program programENTRY ENTRY;; code code entry entry point pointSTART START ADR ADR r1, r1, TEXT TEXT ;; r1 r1 -> ->"Hello World!" World!"LOOP LOOP LDRB LDRB r0, r0, [r1], [r1],#1 #1 ;; get get the the next next byte byteCMP CMP r0, r0, #0 #0 ;; check check for for 'null' 'null' character characterSWINE SWINE SWI_WriteC SWI_WriteC ;; if if not not end, end, print print …. ….BNE BNE LOOP LOOP ;; … and and loop loop back backSWI SWI SWI_Exit SWI_Exit ;; end end of of execution executionTEXT TEXT = "Hello "Hello World!", World!", &0a, &0a,&0d, &0d, 0 ;; string string + CR CR + LF LF + null nullEND ENDAnother Example: Block copy◆Here is another example to block copy from one address (TABLE1)to another (TABLE2), then write it out:AREAAREABlkCpy,BlkCpy,CODE,CODE,READONLYREADONLY;;declaredeclarecodecodeareaareaSWI_WriteCSWI_WriteCEQUEQU&0&0;;outputoutputcharactercharacterininr0r0SWI_ExitSWI_ExitEQUEQU&11&11;;finishfinishprogramprogramENTRYENTRY;;codecodeentryentrypointpointSTARTSTARTADRADRr1,r1,TABLE1TABLE1;;r1r1->->TABLE1TABLE1ADRADRr2,r2,TABLE2TABLE2;;r2r2->->TABLE2TABLE2ADRADRr3,r3,T1ENDT1END;;r3r3->->endendofofTABLE1TABLE1LOOP1LOOP1LDRLDRr0,r0,[r1],[r1],#4#4;;getgetTABLE1TABLE11st1stwordwordSTRSTRr0,r0,[r2],[r2],#4#4;;copycopyintointoTABLE2TABLE2CMPCMPr1,r1,r3r3;;finished?finished?BLTBLTLOOP1LOOP1;;ififnot,not,dodomore,more,elseelseprintprintADRADRr1,r1,TABLE2TABLE2;;r1r1->->TABLE2TABLE2LOOP2LOOP2CMPCMPr0,r0,#0#0;;checkcheckforforendendofoftexttextstringstringSWINESWINESWI_WriteCSWI_WriteC;;ififnotnotend,end,printprint…BNEBNELOOP2LOOP2;;….….andandlooploopbackbackSWISWISWI_ExitSWI_Exit;;finishfinishTABLE1TABLE1=="This"Thisisisthetherightrightstring!",string!",&0a,&0a,&0d,&0d,00T1ENDT1ENDALIGNALIGN;;ensureensurewordwordalignmentalignmentTABLE2TABLE2=="This"Thisisisthethewrongwrongstring!",string!",00ENDENDpykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 11pykc - 28-Jan-00EE2 ComputingSpring Term <strong>Lecture</strong> 5- 12