ftp.barnyard.co.uk/2015.02.ftp.barnyard.co.uk.tar/ftp.barnyard.co.uk/cpm/walnut-creek-CDROM/MBUG/MBUG085.ARC/MCT10-17

Text File | 1979-12-31 | 21KB | 542 lines

Lesson≤ 1░ - 1╖

10«á Conditiona∞ anΣ unconditiona∞áJP≤      E°á- jump≤
11«á Relativσ jump≤                         E° - relativσ jump≤ ▒
                                            E°á- relativσájump≤ ▓
12«á Thσ Stacδ                              E° - pushe≤ anΣ pop≤
13«á Call≤át∩ subroutine≤                   E° - unconditiona∞ call≤ ª ret≤
                                            E° - conditiona∞ call≤
14«á Binar∙ánotatioε                        E° - binar∙ánotatioε
15«á Hexadecima∞ notatioεá                  E°á- he°ánotatioε
16«á Binar∙áCodeΣáDecima∞ánotatioε          E° - BC─ notatioε
17«á Positivσ anΣ negativσ notatioε         E° - +vσ anΣ -vσ number≤


                               INTRODUCTIO╬

Thσ processor¼á unde≥ thσ programmer'≤ direction¼á ha≤ thσ capacit∙ t∩ makσ ì
decisions«á Iεá thσá followinτ lesson≤ wσ shal∞ discus≤ thesσá instruction≤ ì
tha⌠á makσ ß compute≥ morσ thaε jus⌠ ß calculator«á Wσ  shal∞ theε looδá a⌠ ì
number≤ helΣ iε register≤ iε morσ detail.


JUMP╙

Thσá aspec⌠á oµá ß compute≥ tha⌠ make≤ i⌠ morσ thaεá ßá calculato≥á i≤á it≤ ì
abilit∙á t∩á changσá it≤á sequencσ oµ operation≤ a≤á ßá resul⌠á oµá earlie≥ ì
instructions«á Withiε thσ Z8░ therσ i≤ ß counte≥ calleΣ thσ Prograφ Counte≥ ì
o≥á PC«á Thi≤ counte≥ keep≤ tracδ oµ whicΦ instructioε thσ processo≥ i≤á t∩ ì
perforφá next«á B∙ modifyinτ it≤ contents¼á thσ  processo≥ caε bσ jumpeΣ t∩ ì
an∙á instructioε iε memory«á Thσ simples⌠ jum≡ i≤ J╨ nn¼á wherσ nεá i≤á thσ ì
addres≤á t∩ whicΦ thσ  prograφ i≤ t∩ jump«á Thi≤ i≤ aε unconditiona∞á jump¼ ì
thσá Prograφá Counte≥ alway≤ jumpinτ t∩ thσ addres≤ nn«á Thσ morσá powerfu∞ ì
jum≡á i≤á thσ conditiona∞ jump«á Thσ processo≥ test≤ thσ statσ oµá ßá flag¼ ì
beforσá eithe≥á jumpinτ o≥ continuinτ iε sequence«á Thσá conditiona∞á jump≤ ì
involvinτ thσ carr∙ anΣ zer∩ flag≤ are:-

           J╨ NC,nε  jum≡ iµ carr∙ flaτ no⌠ se⌠
           J╨ C,nε   jum≡ iµ carr∙áflaτ set
         á J╨áNZ,nεá jum≡áiµ zer∩ flaτ no⌠ se⌠
           J╨ Z,nε   jum≡áiµázer∩áflaτáse⌠
ì
Indirec⌠á jump≤ arσ als∩ availablσ oε thσ Z8░ usinτ thσ H╠á registe≥á pair« ì
Thσá instructioε J╨ (HL⌐ load≤ thσ prograφ counte≥ witΦ thσ content≤ oµ thσ ì
H╠á register«á Thu≤ thσ addres≤ oµ thσ nex⌠ instructioε t∩ bσ performeΣá i≤ ì
containeΣá withiε thσ H╠ register«á Sincσ thσ content≤ oµ thσá H╠á registe≥ ì
pai≥ caε bσ thσ resul⌠ oµ somσ calculatioε thσ jum≡ caε bσ conditional.

Summar∙ - jump≤
J╨ nε
J╨ (HL⌐
J╨ NC,nε  jum≡ oε n∩ carr∙
J╨ C,nε   jum≡ oε carr∙
J╨ NZ,nε  jum≡ oε no⌠ zer∩
J╨ Z,nε   jum≡áoε zero

EXAMPLE╙áO╞áJUMP╙

AlthougΦá yo⌡ caε ente≥ thσ addres≤ withiε thσ jum≡ iε numbe≥ form¼á iµá i⌠ ì
doe≤á no⌠ equa∞ thσ addres≤ oµ  thσ star⌠ oµ aε instruction¼á thσ assemble≥ ìèwil∞á sto≡á a⌠ thσ JUMP«á Thi≤ i≤ becausσ thσ processo≥ woulΣá perforφá thσ ì
instructioε i⌠ find≤ a⌠ tha⌠ addres≤ anΣ anythinτ coulΣ happeε anΣá usuall∙ ì
does«á T∩á ge⌠ rounΣ thi≤ probleφ  usσ ß label¼á declarinτ i⌠ iε thσá labe∞ ì
columε iε fron⌠ oµ thσ instructioε t∩ whicΦ yo⌡ wisΦ t∩ jump.

      L─ HL,STOR┼
      L─ A,│
LOOP▒ L─ (HL),┴
      IN├ ┴
      C╨ ╕
      J╨ NZ,LOOP▒
LOOP▓ DE├ (HL⌐
      J╨ NZ,LOOP▓
      RE╘
STOR┼ DEF┬ ░


RELATIV┼ JUMP╙

Onσ difficult∙ witΦ thσ abovσ type≤ oµ jum≡ i≤ that¼á iµ thσ star⌠á addres≤ ì
oµá thσá prograφá werσá changed¼á al∞á thσá jum≡á addresse≤á woulΣá requirσ ì
modification«á Relativσá jump≤á d∩ no⌠ havσ thi≤ difficulty«á Thσá absolutσ ì
addres≤á i≤ no⌠ storeΣ witΦ thσ instruction«á I⌠ i≤ thσ differencσá betweeε ì
thσá curren⌠á content≤ oµ thσ prograφ counte≥ anΣ thσ addres≤ t∩á whicΦá wσ ì
wisΦá t∩ jum≡ tha⌠ i≤ storeΣ withiε thσ instruction«á  Relativσá jump≤á arσ ì
limiteΣ t∩ jumpinτ backward≤ 12╕ location≤ anΣ forward≤ 12╖ locations«á Thσ ì
mnemoniπá fo≥  relativσ jump≤ i≤ J╥ anΣ caε takσ al∞ thσ form≤ discusseΣ s∩ ì
fa≥ fo≥ JP¼á i.e« JR¼ J╥ NC¼ J╥ C¼ J╥ N┌ anΣ J╥ Z« Therσ  i≤ onσ particula≥ ì
relativσ jum≡ instructioε tha⌠ i≤ particularl∙ useful¼ namel∙ Decremen⌠ anΣ ì
Jum≡ oε Non-Zero¼ o≥ DJNZ« Thσ ┬ registe≥ i≤ decrementeΣ and¼ iµ thσ resul⌠ ì
i≤á no⌠ zero¼á thσ processo≥ perform≤ ß relativσ jum≡ t∩ thσá computeΣá ne≈ ì
address«á Iµá thσá resul⌠ i≤ zer∩ theε thσ nex⌠ instructioε iε sequencσá i≤ ì
taken« DJN┌ allow≤ u≤ t∩ repea⌠ ß serie≤ oµ instruction≤ ß prese⌠ numbe≥ oµ ì
times«á Thσá numbe≥ oµ repeat≤ i≤ equa∞  t∩ thσ content≤ oµ thσ ┬á registe≥ ì
wheε thσ loo≡ i≤ entered¼á s∩ lonτ a≤ thσ ┬ registe≥ doe≤ no⌠ forφ par⌠á oµ ì
thσ serie≤ oµ inst-ructions« N∩ flag≤ arσ affecteΣ b∙ an∙ jum≡ instruction≤ ì
includinτ DJNZ.

Summar∙á - relativσájump≤
J╥ σ      wherσ σ i≤ thσ displacemen⌠ in
J╥ NC,σ   thσárangσ 12╖ t∩ -12╕
J╥ C,σ
J╥ NZ,σ
J╥ Z,σ
DJN┌ σ    decremen⌠ B anΣ jum≡ oε noεázero


EXAMPL┼áO╞áRELATIV┼ JUMP╙

Al∞á assembler≤á calculatσ thσ displacemen⌠ fo≥á relativσá jumps«á Relativσ ì
jump≤ shoulΣ bσ entereΣ witΦ labels.

      L─  HL,STOR┼
      L─  B,(HL⌐
      IN├ H╠
      L─  E,(HL⌐
      L─  HL,░
      L─  D,░
LOO╨  AD─ HL,D┼è      DJN┌á LOO╨
      L─  (ANS),H╠
      RE╘
STOR┼áDEF┬ ╢
      DEF┬ 5╕
AN╙   DEF╫ ░


EXAMPL┼áO╞ RELATIV┼ JUMP╙

Thi≤ examplσ illustrate≤ ß methoΣ oµ multiplication« Therσ arσ othe≥ faste≥ ì
method≤ tha⌠ wil∞ bσ showε later.

      L─  A,▒
LOOP▒ AD─ A,┴
      L─  (STORE),┴
      J╥  NC,LOOP▒
      IN├ ┴
      L─  B,╖
LOOP▓ AD─ A,┴
      L─  (STORE),┴
      IN├ ┴
      L─  (STORE),┴
      DJN┌ LOOP▓
      RE╘
STOR┼ DEF┬ ░


TH┼ STAC╦

Thσá Stacδ i≤ aε areß oµ externa∞ Randoφ Acces≤ Memory¼á tha⌠ i≤ useΣ a≤á ß ì
serie≤á oµ registe≥ pairs¼á iε conjunctioε witΦ ß Stacδ Pointer«á Thσ Stacδ ì
Pointe≥ (SP⌐ i≤ ß singlσ doublσ lengtΦ registe≥ withiε thσ Z80¼á capablσ oµ ì
holdinτá u≡ t∩ 65335«á Beforσ loadinτ thσ Stacδ froφ an∙ interna∞á registe≥ ì
pair¼á thσ Stacδ Pointe≥ i≤ decrementeΣ twice¼á ¿ thu≤ pointinτ t∩ thσ nex⌠ ì
pai≥ oµ memor∙ locations)« Thσ content≤ oµ thσ registe≥ i≤ theε loadeΣ int∩ ì
thσ Stacδ indirectl∙ usinτ thσ S╨ registe≥ a≤ thσ address«á Thσ instructioε ì
i≤á PUS╚ dd«á Continuinτ t∩ PUS╚ datß oε t∩ thσ Stacδ result≤ iεá thσá datß ì
beinτá pusheΣ iε a⌠ thσ bottoφ oµ thσ Stacδ anΣ  thσ Stacδ Pointe≥á workinτ ì
it≤ wa∙ dowε througΦ memory«á PO╨ dΣ retrieve≤ datß froφ thσ Stacδ int∩ thσ ì
defineΣá registe≥ pair«á  Thσ registe≥ pai≥ i≤ loadeΣ witΦ thσ content≤á oµ ì
thσá memor∙ locatioε pointeΣ t∩ b∙ S╨ register«á Theε thσ S╨ i≤ incrementeΣ ì
twice«á Continuinτ t∩ PO╨ datß froφ thσ Stacδ result≤ iε informatioεá beinτ ì
POPpeΣá froφ thσ bottoφ oµ thσ Stacδ anΣ thσ Stacδ Pointe≥ workinτ it≤á wa∙ ì
bacδá u≡ througΦ thσ memory«á Iµ ß  serie≤ oµ register≤ i≤ PUSHeΣ ont∩á thσ ì
Stack¼á anΣ subsequentl∙ POPpeΣ ofµ thσ Stack¼ the∙ reappea≥ iε thσ reversσ ì
order« Thσ BC¼ D┼ anΣ H╠  register≤ ma∙ bσ PUSHeΣ oε t∩ anΣ POPpeΣ froφ thσ ì
Stack« Thσ Accumulato≥ ma∙ als∩ bσ PUSHeΣ oε t∩ thσ Stack¼ bu⌠ thi≤ i≤ donσ ì
witΦ thσ flaτ registe≥ F¼á whicΦ wil∞ bσ discusseΣ later« Thσ Stacδ Pointe≥ ì
caεá takσá par⌠á iε ß numbe≥ oµ instruction≤ alread∙á discussed«á The∙á arσ ì
simpl∙ thosσ iε  whicΦ DE¼á o≥ B├ caε takσ part« Thσ instructioε equivalen⌠ ì
t∩á E╪á DE,H╠ i≤ howeve≥ E╪ (SP),HL¼á tha⌠ EXchange≤ thσá content≤á oµá thσ ì
bottoφ oµ thσ Stacδ witΦ thσ H╠ register.

Summar∙ - thσ Stacδ anΣ S╨ registe≥
PUS╚ D─  wherσ D─ i≤ AF¼áBC¼áDE¼ o≥ HL«
         Froφ no≈ oε dΣ caε bσ considereΣáa≤ representinτ BC¼áDE¼áHL o≥ SP«á
PO╨  D─
L─   SP,nεèL─   SP,(nn⌐
L─   (nn),S╨
L─   SP,H╠
AD─  HL,S╨
AD├  HL,S╨
SB├  HL,S╨
IN├  S╨
DE├  S╨
E╪   (SP),HL


EXAMPLE╙ USIN╟ TH┼ STAC╦

Thσ Stacδ Pointe≥ i≤ initiall∙ se⌠ to¼á fo≥ example¼ 409┤ anΣ work≤ it≤ wa∙ ì
dowεá toward≤ thσ prograφ area«á Kee≡ thσ S╨ a⌠ thσ to≡ enΣ oµ thi≤ areß oµ ì
memor∙ t∩ ensurσ tha⌠ thσ assemble≥ doe≤ no⌠ stop«á Thσ actua∞ valuσ iε thσ ì
S╨ registe≥ i≤ immateria∞ fo≥ mos⌠ oµ it≤ applications.

     L─   HL,5678╣
     L─   DE,3456╖
     L─   BC,1234╡
     PUS╚ H╠
     PUS╚ B├
     PUS╚áD┼
     E╪á  (SP),H╠
     L─   BC,░
     PO╨  D┼
     PO╨  B├
     PO╨  H╠
     RE╘


CALL╙áT╧ SUBROUTINE╙

┴á subroutinσ i≤ ß par⌠ oµ ß prograφ tha⌠ i≤ calleΣ froφ thσá maiεá prograφ ì
anΣá oncσá completeΣ return≤ bacδ t∩ thσ maiε program«á  Thσ subroutinσá i≤ ì
usuall∙ calleΣ froφ ß numbe≥ oµ place≤ iε thσ program«á Thσ Z8░ allow≤ thi≤ ì
featurσ b∙ thσ instructioε CALL«á Thσ CAL╠ instructioε i≤ simila≥ t∩ thσ J╨ ì
instructioεá excep⌠á tha⌠ thσ addres≤ oµ thσ nex⌠á sequentia∞á instruction¼ ì
helΣ iε thσ P├ register¼á i≤ PUSHeΣ ont∩ thσ Stack¼á prio≥ t∩ thσ jump« Thσ ì
returε instructioε RET¼á POP≤ thσ returε addres≤ froφ thσ Stack¼á bacδ int∩ ì
thσá P├á register¼á t∩ continuσ wherσ  thσ maiε prograφ lef⌠á off«á Iµá thσ ì
numbe≥á oµ PUSHe≤ anΣ CALL≤ iε thσ subroutinσ doe≤ no⌠ equa∞ thσ numbe≥á oµ ì
POP≤á anΣ RETurns¼á obviousl∙ thi≤ wil∞ no⌠ happen«á BotΦá conditiona∞á anΣ ì
unconditiona∞ CALL≤ anΣ RETurn≤ arσ availablσ oε thσ Z80« Al∞ fou≥ type≤ oµ ì
condition≤á alread∙ considereΣ ¿ NC¼á C¼á N┌ anΣ ┌ ⌐ caε bσ useΣ witΦ CALL≤ ì
anΣ RETurns.

Summar∙ - call≤ anΣáreturn≤
     CAL╠á NC,nε
     RE╘   N├á      N∩áCarr∙
 á   CAL╠á C,nε
     RE╘á  ├á       Carr∙áse⌠
     CAL╠  NZ,nε
     RE╘   N┌á      No⌠áZer∩
 á   CAL╠á Z,nε
     RE╘á  ┌        Zer∩ se⌠

Iµá thσ conditioε i≤ no⌠ met¼á thσ prograφ wil∞ no⌠ CAL╠ thσ subroutinσá o≥ ìèRETurε froφ it.


EXAMPL┼ O╞ CALL╙ T╧áSUBROUTIN┼

I⌠á mus⌠ bσ remembereΣ tha⌠ ß subroutinσ ma∙ requirσ thσ usσ oµ ßá registe≥ ì
whosσá content≤ arσ stil∞ requireΣ iε thσ maiε program«á It≤ content≤á mus⌠ ì
thereforσá bσá PUSHeΣ ont∩ thσ stacδ o≥á otherwisσá storeΣá elsewhere¼á anΣ ì
retrieveΣ a⌠ thσ enΣ oµ thσ subroutine« Thσ followinτ examplσ add≤ togethe≥ ì
thσ firs⌠ 3▓ pair≤ oµ memor∙ locations« Thσ resul⌠ i≤ lef⌠ iε HL« Thσ stacδ ì
anΣá thσ H╠ register≤ arσ botΦ useΣ a≤ storagσ inspitσ oµ thσá interferencσ ì
oµ thσ CALL¼ b∙ usinτ thσ E╪ (SP),H╠ instruction:

       L─   HL,░
       L─   B,3▓
       PUS╚ H╠
LOOP▒á L─á  E,(HL⌐
       IN├  H╠
       L─   D,(HL⌐
       IN├  H╠
       E╪á  (SP),H╠
       CAL╠ SUB╥
       E╪   (SP),H╠
       DJN┌ LOOP▒
       PO╨  H╠
       RE╘
SUB╥   AD─  HL,D┼
       RE╘


EXAMPL┼áO╞ CONDITIONA╠ CALL╙

Thi≤á simplσá examplσá count≤ u≡ thσ numbe≥ oµ memor∙ location≤á holdinτá ß ì
numbe≥ abovσ 19▓ anΣ belo≈ 6┤ iε thσ firs⌠ 3▓ memor∙ locations.

      L─   HL,░
      L─   DE,░
      L─   B,3▓
LOO╨  L─   A,(HL⌐
      C╨   19│
      CAL╠ NC,ABOV┼
      C╨á  6┤
      CAL╠ C,BELO╫
      IN├  H╠
      DJN┌ LOO╨
      RE╘
ABOV┼ IN├  ┼
      RE╘
BELO╫áIN├ á─
      RE╘





BINAR┘ NOTATIO╬

Iµ wσ werσ t∩ takσ thσ to≡ ofµ ß Z8░ chi≡ and¼ usinτ ß powerfu∞ microscope¼ ì
looδ a⌠ ß register¼á wσ woulΣ seσ tha⌠ i⌠ consist≤ oµ ╕ cells«á Thesσ cell≤ ìèarσ capablσ oµ beinτ switcheΣ betweeε tw∩ differen⌠ states«á Conventionall∙ ì
wσ designatσ onσ statσ a≤ "1ó anΣ thσ othe≥ a≤ "0"«á Thσ patterε oµ 1'≤ anΣ ì
0'≤ coulΣ bσ 1011100░ say« Ho≈ d∩ wσ interpre⌠ thi≤ pattern┐ Thσ answe≥ is¼ ì
anywa∙ wσ like« Iε thi≤ lessoε wσ sho≈ tha⌠ thi≤  patterε coulΣ bσ 184,-72¼ ì
B8╚á anΣá eveε C╨ B«á Thσ onl∙ onσ thσ processo≥ ha≤ buil⌠ int∩ i⌠á i≤á thσ ì
latter«á EacΦ digi⌠ iε ß numbe≥ caε bσ considereΣ t∩ havσ ß weighting«á Fo≥ ì
decima∞ number≤ thesσ weighting≤ arσ Digi⌠ -

                │    ▓    ▒    ░
             100░  10░   1░    ▒

sincσá eacΦá digi⌠á caε havσ onσ oµ teε states¼á designateΣá ░á - 9«á Iεá ß ì
registe≥ eacΦ digi⌠ o≥ bi⌠ ha≤ onl∙ tw∩ states«á Thereforσ wσ caε interpre⌠ ì
thσ digit≤ a≤ havinτ weighting≤ oµ bi⌠ -

             ╖    ╢   ╡    ┤   │   ▓   ▒   ░
           12╕á  6┤  3▓á  1╢   ╕   ┤   ▓   ▒

Takinτá ou≥ numbe≥ 1011100░ fo≥ examplσ anΣ notinτ tha⌠ bit≤ tha⌠á arσá "0ó ì
contributσá nothinτá t∩ thσ number¼á thσ decima∞ equivalen⌠ oµ 1011100░á i≤ ì
giveε b∙ -

                  128+32+16+8=184

Thσ maximuφ numbe≥ tha⌠ caε bσ helΣ iε aε ╕ bi⌠ registe≥ i≤ -
ì
                  128+64+32+16+8+4+2+1=25╡

Thi≤á i≤á thσá numbe≥á wσá originall∙á assumeΣá a⌠á thσá beginninτá oµá ou≥ ì
discussion«á Jus⌠á a≤á wσ adΣ tw∩ decima∞ number≤á together¼á carryinτá onσ ì
forwarΣ iµ thσ suφ oµ tw∩ digit≤ i≤ greate≥ thaε 9¼á wσ caε adΣ tw∩á binar∙ ì
numbers« Iµ thσ suφ i≤ abovσ one¼ ß onσ i≤ carrieΣ forward« ie.-

                  ░ ▒ ▒ ░ ▒ ▒ ▒ ░
              ½á  ░á░á▒á░ ░ ▒ ░ ▒
                ------------------
       give≤      ▒ ░ ░ ▒ ░ ░ ▒ ▒

Simila≥ wσ caε perforφ subtraction« i.e.

                  ░ ▒ ▒ ░ ░á▒á▒á░
               -  ░á░á▒á░ ▒ ▒ ░ ▒
                -------------------
       give≤      ░ ░ ▒ ▒ ▒ ░ ░ 1


EXAMPLE╙áO╞áBINAR┘ NOTATIO╬

Somσá assembler≤á havσ onσ instructioε no⌠ normall∙ founΣ iεá most¼á namel∙ ì
BIN«á Thσ assemble≥ allocate≤ onσ memor∙ locatioε t∩ thσ line¼á whicΦ woulΣ ì
normall∙ bσ labelled«á I⌠ constantl∙ display≤ thi≤ locatioε iε binar∙ form« ì
Thσá numbe≥ afte≥ BI╬ i≤ thσ numbe≥ initiall∙ inserteΣ int∩á thσá location« ì
Afte≥ goinτ througΦ thσ   followinτ examplσ tr∙ modifyinτ thσ displayinτ oµ ì
STOR┼ iε examplσ 11B.

      L─  HL,STOR┼
LOO╨  DE├ (HL⌐
      J╥  NZ,LOO╨
      L─  A,(NUM1⌐è      L─  HL,NUM▓
      AD─ A,(HL⌐
      L─  (ANS),┴
      RE╘
STOR┼áBI╬á 1╡
NUM▒  BI╬  8▒
NUM▓á BI╬á 9╡
AN╙á  BI╬á ░


HEXADECIMA╠áNOTATIO╬

Binar∙á number≤ arσ to∩ cumbersomσ anΣ decima∞ to∩ inconvenien⌠ t∩á displa∙ ì
multiplσá registe≥ numbers«á ┴ methoΣ oµ presentinτ binar∙ number≤ ha≤ beeε ì
developeΣ t∩ ge⌠ rounΣ thesσ problems«á InsteaΣ oµ basinτ thσ numbe≥ oεá 1░ ì
a≤ iε decima∞ o≥ ▓ a≤ iε binary¼á wσ basσ i⌠ oε 16«á  Thi≤ numberinτ systeφ ì
i≤á calleΣá Hexadecimal«á Wσ requirσ 1╢ differen⌠  symbol≤ t∩á represen⌠á ß ì
digit«á 0-╣á give≤ u≤ thσ firs⌠ 1░ anΣ A-╞ thσ fina∞ 6«á ┴ lis⌠ oµ decimal¼ ì
binary¼ anΣ hexadecima∞ equivalent≤ arσ giveε below:-

              Decima∞   Binar∙á    Hexadecima∞
         á       ░       000░á         ░
         á       ▒       000▒          ▒
                 ▓á      001░á         ▓
                 │á      001▒á         │
         á       ┤       010░á         ┤
         á       ╡á      010▒          ╡
        áá       ╢á      011░á         ╢
         á       ╖á      011▒á         ╖
        áá       ╕       100░á         ╕
         á       ╣       100▒          ╣
                1░á      101░á         ┴
                1▒á      101▒á         ┬
        á       1▓       110░á         ├
        á       1│á      110▒          ─
                1┤       111░          ┼
                1╡       111▒          F

Al∞á 1╢ state≤ oµ thσ ┤ binar∙ bit≤ arσ covereΣ b∙ 0-F«á Hencσ onl∙ ▓á hex-ì
adecima∞ digit≤ arσ requireΣ t∩ displa∙ thσ content≤ oµ aε ╕ bi⌠á register« ì
Ou≥ examplσ -

          1011100░áá divide≤áu≡áinto  101▒á100░á o≥áB╕áHex«

┴ doublσ registe≥ holdinτ sa∙ 1100100▒ 1011100░ wil∞ bσ representeΣ b∙ C9B╕ ì
hex«á Conversioεá oµá ßá he° numbe≥ t∩ it≤ decima∞ equivalen⌠á follow≤á thσ ì
standarΣ calculation« i.e.

á  B╕á he°á ╜ (┬ hex) ¬ 16 ½á(8) ¬á▒
á           ╜ 11*16 + 8
            = 18┤

Alway≤á remembe≥á tha⌠ ß he° numbe≥ ha≤ ß basσ oµá 1╢á decimal¼á ßá decima∞ ì
numbe≥ ha≤ ß basσ oµ 1░ anΣ ß binar∙ numbe≥ ß basσ oµ 2.

EXAMPLE╙áO╞áHEXADECIMA╠ NOTATIO╬

No≈á tha⌠ he° notatioε ha≤ beeε introduceΣ wσ caε discus≤ ho≈ thσá compute≥ ì
store≤ program≤ iε morσ detail«á A≤ alread∙ mentioned¼á i⌠ storeΣ memor∙ a≤ ìènumbers«á Somσá instruction≤ onl∙ requirσ ß singlσ number¼á other≤á requirσ ì
more.

     L─  A,25╚
     AD─ A,┴
     DA┴
     L─  HL,4589╚
     L─  DE,3812╚
     L─  A,╠
     SU┬ ┼
     DA┴
     L─  L,┴
     L─  A,╚
     SB├ A,─
     DA┴
     L─  H,┴
     RE╘


BINAR┘ CODE─áDECIMA╠áNOTATIO╬

Wσá woulΣ stil∞ likσ t∩ bσ ablσ t∩ perforφ calculation≤ oε decima∞á number≤ ì
withou⌠á havinτ t∩ conver⌠ t∩ binar∙ o≥ hexadecima∞ form«á Hexidecima∞ forφ ì
hold≤á ßá ke∙á t∩á ß methoΣ oµá achievinτá this«á Thσá he°á digi⌠á require≤ ì
adjustmen⌠ t∩ avoiΣ thσ digit≤ A-F« Addinτ ╢ t∩ thσ digit¼ iµ thesσ symbol≤ ì
appear¼á result≤ iε correc⌠ adjustment« Takσ  thσ he° numbe≥ ├ (╜ 1▓ dec.)« ì
Addinτá ╢ give≤ 1╕ decima∞ o≥ 1▓ hex«á Hencσ thσ he° numbe≥ look≤ identica∞ ì
t∩á thσá decima∞á answe≥ required«á EacΦ digi⌠ i≤ helΣ iεá ┤á bit≤á oµá thσ ì
register«á Thesσ ┤ bit≤ arσ calleΣ ß nibble¼ thσ wholσ ╕ bi⌠ worΣ i≤ termeΣ ì
ß byte«á Wh∩ saiΣ programmer≤ havσ n∩ sensσ oµ humour┐ Thi≤ presentatioε i≤ ì
termeΣá Binar∙á CodeΣá Decima∞á o≥ BCD«á Thσá Z8░á accommodate≤á thσá abovσ ì
adjustment¼ witΦ thσ instructioε   DAA¼ o≥ Decima∞ Adjus⌠ Accumulator« Thi≤ ì
instructioεá i≤ useΣ afte≥ aε ╕ bi⌠ ADD¼á ADC¼á SUB¼á o≥ SB├ t∩ adjus⌠á thσ ì
Accumulato≥á content≤á b∙ addinτ ╢ t∩ thσ nibble¼á iµ eithe≥ nibblσ iεá thσ ì
answe≥á i≤á abovσ 9«á T∩ perforφ thi≤ operatioε thσ Z8░á use≤á tw∩á furthe≥ ì
flags¼á thσ ╚ flaτ ¿ ß Halµ carr∙  betweeε nibble≤ )¼á anΣ ╬ flaτ (subtrac⌠ ì
instructioε performeΣ last)«á Sincσ thesσ flag≤ canno⌠ bσ testeΣ a≤ par⌠ oµ ì
ß conditiona∞ jum≡ etc« the∙ arσ oµ littlσ use.


EXAMPLE╙ O╞ B.C.D« NOTATIO╬

Thesσ example≤ sho≈ simplσ additioε anΣ subtractioε iε BC─ format« Remembe≥ ì
t∩ kee≡ iε He° displa∙ modσ o≥ thσ illustration≤ wil∞ no⌠ appea≥ iε BCD.

      L─  A,25╚
      L─  HL,STOR┼
      L─  DE,2468╚
      IN├ (HL⌐
      RE╘
      BI╬ 65╚
      BI╬ 87╚
      BI╬á9A╚
      BI╬ DF╚
      BI╬áFF╚
      BI╬ 64╚
STOR┼ DEF┬ 6┤

èPOSITIV┼ AN─áNEGATIV┼áNUMBE╥ NOTATIO╬

S∩á fa≥ wσ havσ onl∙ deal⌠ witΦ positivσ numbers«á Iµ wσ haΣ takeε ╢ froφ ╡ ì
wσá woulΣá enΣá u≡á witΦá thσ answe≥á 25╡á witΦá carr∙á set«á Therσá i≤á aε ì
interpretatioεá oµá number≤ tha⌠ allow≤ u≤ t∩ conside≥ thi≤ answe≥á a≤á thσ ì
negativσá numbe≥ -1«á Iε thi≤ interpretatioε -▒ mus⌠ equa∞ 25╡ o≥ 11111111¼ ì
o≥á thσá processo≥á woulΣ no⌠ bσ ablσ t∩ takσ ╢ froφ ╡ anΣá ge⌠á thσá righ⌠ ì
answer«á Furthe≥ 5-╖ ╜ -▓ o≥ 25┤ o≥ 11111110«á Bi⌠ 7¼á thσ mos⌠ significan⌠ ì
bit¼á represent≤ thσ sigε ¿ ½ o≥ - ⌐ oµ thσ number«á Wheε bi⌠ ╖ i≤ "1óá thσ ì
numbe≥á i≤á negative¼á anΣá wheεá "0ó thσ numbe≥á i≤á positive«á ┴á  usefu∞ ì
operatioε woulΣ bσ t∩ makσ ß positivσ numbe≥ negative«á Changinτ bi⌠ ╖ doe≤ ì
no⌠á d∩á this«á Takσá thσ positivσ numbe≥ ▓ o≥ 0000001░á anΣá it≤á negativσ ì
equivalen⌠á -▓á o≥ 1111111░ fo≥ example«á Invertinτ al∞ bit≤ oµ thσá binar∙ ì
numbe≥ oµ +▓  give≤ 11111101¼á whicΦ i≤ ▒ les≤ thaε tha⌠ fo≥ -2«á Hencσá t∩ ì
makσá ßá positivσ numbe≥ negativσ wσ inver⌠ al∞ it≤ binar∙ bits¼á knowεá a≤ ì
Complimenting¼á anΣ adΣ 1«á Thσ notatioε i≤ generall∙ termeΣ 2≤ complement«  ì
i.e.

                   ░ ░ ░ ░ ░ ▒ ░ ▒    = +╡
          inver⌠   ▒ ▒ ▒ ▒ ▒ ░ ▒ ░
        adΣ ▒      ▒ ▒ ▒ ▒ ▒ ░ ▒ ▒    = -╡
          inver⌠á  ░á░ ░ ░ ░ ▒ ░ ░
        adΣ ▒      ░ ░ ░ ░ ░ ▒ ░ ▒    = +╡

Thσ operatioε luckil∙ als∩ make≤ ß negativσ numbe≥ positive« ┴ ver∙ importì
an⌠á point«á Thσ larges⌠ positivσ numbe≥ wσ caε holΣ iε ß singlσá register¼  ì
usinτá thi≤á notatioε i≤ 0111111▒ o≥ 12╖ anΣ thσá larges⌠á negativσá numbe≥ ì
1000000░á o≥ -128«á Thσ Z8░ ha≤ tw∩ instruction≤ tha⌠ ma∙ bσ useΣ fo≥ thesσ ì
operation≤ CP╠ complement≤ o≥ invert≤ thσ content≤ oµ thσ Accumulator«á NE╟ ì
negates¼áá o≥áá make≤á negative¼áá thσá content≤á oµá thσá Accumulato≥áá b∙ ì
complementinτ anΣ addinτ ▒ iε onσ operation« ┴ Sigε flaτ (S⌐ i≤ provideΣ oε ì
thσá Z80«á I⌠á duplicate≤á thσá sigε ¿ bi⌠ ╖ ⌐á oµá thσá answe≥á afte≥á an∙ ì
arithmetiπá operatioεá oεá thσá Accumulator«á Thσ sigε flaτ i≤á "0óá fo≥á ß ì
positivσ resul⌠ anΣ "1ó fo≥ ß negativσ result«á Wheε performinτá arithmetiπ ì
operation≤á whicΦ wσ wisΦ t∩ interpre⌠ withiε thσ rangσ -12╕ t∩á +127¼á thσ ì
Carr∙ flaτ n∩ longe≥ signal≤ aε ou⌠ oµ rangσ result«á Howeve≥ anothe≥ flag¼ ì
thσá overflo≈ (P/V⌐ flaτ does«á I⌠ i≤ ß "1ó wheneve≥ thσ answe≥ i≤á outsidσ ì
thσá rangσá -12╕ t∩ +12╖ anΣ "0ó withiε thσ   range«á I⌠ i≤á effectivel∙á ß ì
carr∙á int∩á bi⌠á ╖ whicΦ woulΣ makσ thσ sigε bi⌠ incorrect«á Therσá i≤á n∩ ì
reasoε fo≥ stickinτ t∩ ╕ bit≤ usinτ thi≤ method«á S∩ lonτ a≤ thσ numbe≥á oµ ì
bit≤á i≤á sufficien⌠ anΣ thσ mos⌠ significan⌠ bi⌠ i≤ takeε a≤ thσ sigεá bi⌠ ì
an∙á sizσá positivσ o≥ negativσ numbe≥ caεá bσá represented«á Performinτá ß ì
simila≥á calculatioεá t∩á thσ abovσ wσ caε sho≈ tha⌠ ßá registe≥á pai≥á caε ì
represen⌠á ß numbe≥ iε thσ  rangσ +3276╖ t∩ -32768«á Thσ Sigε anΣá overflo≈ ì
flag≤á arσ als∩ operativσ afte≥ AD├ anΣ SB├ instructioε oε thσ H╠ register« ì
I⌠á reflect≤á thσ 15tΦ bi⌠ (o≥ bi⌠ ╖ oµ ╚ register)«á I⌠á i≤á importan⌠á t∩ ì
remembe≥á tha⌠á thσá Zero¼á Carry¼á Sign¼á anΣ overflo≈á flag≤á arσá alway≤ ì
operativσá afte≥á aεá ╕  bi⌠ arithmetiπ instructioε o≥ ßá 1╢á bi⌠á additioε ì
involvinτ thσ Carry«á You≥ interpretatioε oµ thσ resul⌠ determine≤ iε whicΦ ì
flag(s⌐ yo⌡ shoulΣ bσ interesteΣ in.





EXAMPLE╙ O╞á+V┼áAN─á-V┼áNOTATIO╬

AlthougΦá thesσá example≤á appea≥ t∩ bσ al∞ positive¼á the∙ caεá bσá vieweΣ ì
equall∙á a≤á negativσá wherσ appropriatσ anΣ thσ Sigεá anΣá overflo≈á flag≤ ìèobserveΣ operating«á Remembe≥ ß numbe≥ abovσ 12╕ i≤ negative« Sincσ al∞ thσ ì
negativσá number≤ iε thσ example≤ arσ small¼á the∙ caε bσ seeε quickl∙á anΣ ì
simpl∙á bσá converteΣá b∙ subtractinτ thσá numbe≥á froφá 256«á i.e«á -▓á i≤ ì
equivalen⌠ t∩ 25┤ etc.

      L─  A,25┤
      NE╟
      NE╟
      AD─ A,▓
      IN├ ┴á
      L─á HL,▓
      L─  DE,FFFE╚
      AD─áHL,D┼
      CP╠ L─ HL,STOR┼
      L─  (HL),7F╚
      IN├ (HL)
      RE╘
STOR┼ BI╬ 0