The 8080 Microprocessor, By Dave Hardy (Technical CBBS : 313 846-6127 ) Th Inte 808 wa th firs microprocesso t gai acceptanc i th microcompute fiel an did i fact hel creat th field I i currentl th mos widel use microprocesso simpl becaus i wa firs t reac th marketplace Thi situatio i rapidl changin a superio product suc a th 808 an Z8 ar gainin i popularity. Th 8080 fro a architectura standpoint i quit primitive an improve processor ar bein designe int ne products bu du t it wid acceptance multipl sources an larg lin o suppor chips th 808 i her t stay. PURPOSE. Th 808 instructio se reflect th contro natur o thi microprocessor I i heavil loade wit dat transfe instruction an ha I an OU instruction fo inpu an outpu control Conditiona jumps subroutin calls an subroutin return (wit parit eve o od jum instructions ar mean t b use t simplif dat handling. Th 808 ha fe dat processin characteristics bu som importan feature ar missing Som o th EDP-typ instruction includ th AD an SUBTRAC instructions th DOUBL PRECISIO AD instructio (althoug thi featur i primaril intende fo addres manipulations) an th DECIMA ARITHMETI ADJUS instruction Lackin ar th arithmeti shif instructions signe overflo detection an th arithmeti conditiona branc instruction suc a BRANC GREATE THA O EQUA TO Thes shortcoming coul no b easil overcom becaus o simpl fac neve mentione i mos manufacturer literature th 808 i no designe t d signe twos complemen arithmetic Th modularit o th twos complemen numberin syste make man twos complemen operation wor (e.g - 6) bu th 808 ha n wa o tellin i resul i negativ o positive goo exampl o thi i th compariso o tw number t se whic i larger compar instructio followe b a examinatio o th overflo bi ca b performe t compar pai o twos complemen numbers se overflo bi mean tha th registe bein compare t th accumulato wa greate tha th accumulato value bu onl i th value wer o lik sign I th register wer o differen sign th se overflo bi woul hav jus th opposit meaning On mus therefor manuall kee trac o th sign o number i th program This o course involve additiona softwar an executio time. FEATURES. Th 808 ha register-oriente architectur containin si 8-bi register tha ma b use individuall o i pair fo 8- an 16-bi operations A accumulato i provide t ac a primar workin register. 1 The 8080 Microprocessor, By Dave Hardy (Technical CBBS : 313 846-6127 ) 8080 Programming Model Accumulator Flag Register _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |_|_|_|_|_|_|_|_| |_|_|_|_|_|_|_|_| 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 'B' Register 'C' Register _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |_|_|_|_|_|_|_|_| |_|_|_|_|_|_|_|_| 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 'D' Register 'E' Register _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |_|_|_|_|_|_|_|_| |_|_|_|_|_|_|_|_| 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 'H' Register 'L' Register _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |_|_|_|_|_|_|_|_| |_|_|_|_|_|_|_|_| 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 Program Counter __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ |__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__| 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Stack Pointer __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ |__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__| 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Th 808 i als capabl o stac operations a th instructio se indicates separat 16-bi stac pointe keep trac o th push-dow stac tha reside i th combine progra an dat memory Th stac i usefu fo implementin subroutines Becaus subroutin retur addresse ar automaticall pushe dow ont th stac b th subroutin call subroutin nestin i limite onl b th amoun o read-onl memor use ha provided Thi versatil featur make mor structure program possible an lend itsel t ver comple contro task an dat processing. Al th 808 feature mentione s fa indicat tha th 808 i wel suite fo contro tasks Th interrup an DM capabilitie o thi chi confir thi suspicion A asynchronou vectore interrup capabilit allow externa device operatin throug a 8080-serie suppor chi (th 825 priorit interrup controller) t interrup progra executio an vecto th progra t a appropriat servic routine Man device ca efficientl mov dat i t an ou o memor usin th 8080' DM capabilit an anothe suppor chip th 825 DM controller. 2 The 8080 Microprocessor, By Dave Hardy (Technical CBBS : 313 846-6127 ) Not tha thi i reall 3-chi microprocessor tha is i take a leas thre chip o correspondin discret hardwar t buil usefu microprocessor I additio t th 8080 a 822 cloc generator-drive an a 822 bidirectiona bu drive ar needed Upgrade fro th 808 microprocesso hav th function o th tw additiona chip buil int singl package. INSTRUCTIONS. Th 808 ha abou 10 instructions Th exac coun varie fo eac manufacture du t th wa instruction ar counted no becaus o difference betwee th microprocessors Instruction var fro t byte i length dependin o th addressin mod o th instruction Th instructio se ca b broke int seve primar group a show below. DAT TRANSFE INSTRUCTION mov 8-bi dat fro registe t register an als t memory fe 16-bi dat transfe instruction ar provide a well A interestin XCH instructio swap tw specifi registe pair (th D register wit th H registers) Tw 16-bi transfe instruction (th LHL an SHL instructions mov dat fro registe pai t memory o fro memor t registe pair bu thes operation ca onl b performe o th H registe pair Th I/ transfe instruction (I an OUT ar provide fo simpl I/ handling. CONTRO INSTRUCTION (ther ar six ar th usua standards an includ th NO (no-operation) HL (halt) an interrup enabl an disabl instructions. BRANC INSTRUCTION (29 conditiona an unconditional a firs glanc see overwhelming bu close investigatio reveal tha branchin i onl allowe o fou conditions carry zero sign an parity N combine conditiona branche suc a "branch-less-than ar provided Thi make signe comparison difficult. ARITHMETI INSTRUCTION lik th transfe instruction includ 8- an 16-bi operations Th basi ADD' an SUBTRACT' ar presen plu DECIMA ARITHMETI ADJUS instructio i included Again certai operation onl appl t certai registers An registe ca b adde t th accumulator bu memor canno b adde t registe withou firs goin throug th accumulator Tw arithmeti instruction particularl suite t multiple-precisio arithmeti ar included th AD WIT CARRY an SUBTRAC WIT BORRO instructions. STAC INSTRUCTION wor wit 16-bi registe pair rathe tha individua registers Pushin tw register ont th stac a onc make fas machin statu savin possible. LOGICA INSTRUCTION ar simila t th arithmeti operation excep tha onl 8-bi logica operation ar officiall possible On 16-bi logica operation however i possibl - althoug no immediatel apparen fro th instructio set B usin th 16-bi AD instruction th H registe pai ca b adde t itself resultin i HL*2 o a arithmeti lef shift Man 808 programmer us thi trick especiall whe workin wit double-precisio numbers. 3 The 8080 Microprocessor, By Dave Hardy (Technical CBBS : 313 846-6127 ) INCREMENT/DECREMEN INSTRUCTION ar provide fo 1 discret operations Upo clos examination w se tha statu flag ar se fo 8-bi registe increment an decrements bu n flag ar se fo 16-bi ones Thi mod o operatio wa chose s 8-bi dat statu woul no b destroye b addres manipulation 16-bi increment an decrement ar typicall employe whe usin indexe addressin throug th H registe pair Wit th curren statu settin method th 8-bi arithmeti operation statu ca b save whil obtainin th nex dat byt throug indexe addressing Thi i particularl usefu fo multiple-precisio arithmeti i whic th carr fro leas significan AD ca b save an adde wit th mos significan AD throug incremente inde addressing Th mai proble wit th conditiona statu settin arrangemen i tha i limit th capabilitie o th 16-bi registe pairs. Th 808 ha fou basi addressin modes Direc mod addressin allow direc loadin o storin o th accumulato o H registe pai fro th addres specifie i th tw byte followin th instruction Th immediat addressin mod allow th loadin o an registe o registe pai wit th dat followin th instruction Implie addressin i use i operation needin n memor referenc (suc a register-to-registe transfers) Indexe addressin allow th content o th BC DE o H registe pair t b use a 16-bi pointer t th dat bein loade o store i memory Auto-incremen an auto-decremen addressin o th stac pointe ar als provide b th stac operations. INTERFAC CHARACTERISTICS. Th inpu an outpu instruction simpl pu a 8-bi devic cod o th 16-bi addres bu (repeate i bot th hig an lo orde addres bytes) Th outpu instructio put th presen accumulato dat o th dat bus an th inpu instructio clock whateve i o th dat bu int th accumulator Statu line indicat whe I/ operation ar bein performed Device o th bu ca us thes line t loa devic register o driv th dat bus Becaus addres line an dat line ar availabl outsid th processor memory-mappe I/ i als possible B buildin a interfac devic tha act a memor locatio a certai address dat ca b sen t tha devic simpl b writin int th device' assigne memor location Th 808 contro line ar configure fo simpl direc memor acces interfacing Th processo ca g int hol stat whe DM devic applie signa t th 8080' HOL line Th 808 promptl disconnect itsel fro th bus allowin th DM devic t tak ove addres an dat lin control an acces memor directly. SUPPORT. Th 808 i currentl on o th bes supporte microprocessor i regar t interfac chips Becaus eigh manufacturer second-sourc th part everythin fro DM controller t floppy-dis controller i available. Softwar suppor i equall diversified Editors assemblers an high-orde language ar available no t mentio th popula CP/ operatin system. 4 The 8080 Microprocessor, By Dave Hardy (Technical CBBS : 313 846-6127 ) PHYSICA CHARACTERISTICS. Th 808 i availabl i man cloc speed rangin fro t Mhz Du t th intens competitio i th 808 market price ar ver low bu th fac tha th 822 an 822 suppor chip ar neede t mak th 808 operationa add significantl t thi cost. Th 808 i a NMO microprocessor require thre D voltage fo power an dissipate abou watt Again cloc spee an versio mak bi difference. TH 8085 TH 808 i a upgrade versio o th 808 tha incorporate built-i cloc an syste controller thu eliminatin th nee fo th tw 808 suppor chip (th 822 cloc generato an 822 syste controller) I i softwar compatibl wit th 808 an contain tw additiona instructions RI an SI fo interrup mas contro an limite seria I/O. Althoug softwar compatibl wit th 8080 instructio executio time ar differen fro thos o th 8080 an recen change o th 808 productio mask b Inte mean tha no eve al 8085' hav th sam characteristics. Man o th "undocumented 808 instruction tha ar i widesprea us ar n longe vali becaus o th ne mask s us o an instruction no specificall documente b th chip' manufacture i n longe goo ide fo th 8085. 5