Page 395 - FUNDAMENTALS OF COMPUTER

NPP

NPP CPU Architecture, Addressing Modes and Data Transfer Schemes 395

DEC m: Decrement the contents of memory DEC m: S>mQ>m goJ_oÝQ> _| _o_moar cmoHo$eZ m Ho$
location m in data segment by 1 and put the H$ÝQ>oÝQ²>g _| 1 KQ>mVm h¡ VWm n[aUm_ H$mo Cgr cmoHo$eZ
result in the same location:
na aIVm h¡&
 Assembler Directives can be used to · Agoå~ca S>mBaopŠQ>ìg g§Ho$V H$aZo Ho$ {cE Cn`moJ
indicate whether memory points to a byte {H$`m Om gH$Vm h¡ {H$ Š`m _o_moar EH$ ~mBQ> `m EH$
or to a word:
dS>© H$mo B§{JV H$aVm h¡&
e.g. DEC BYTE PTR [BX]
DEC WORD PTR [BX]

 CF is not affected. · CF à^m{dV Zht hmoVm h¡&
SUB r , r : Subtract contents of register r 2 SUB r , r : a{OñQ>a r Ho$ H$ÝQ>oÝQ²>g H$mo a{OñQ>a
2
1
2
2
1
from r and put the result in r . Thus r is the r go KQ>mVm h¡ VWm n[aUm_ H$mo r _| aIVm h¡& Bg àH$ma
1
1
1
1
destination Register. r S>opñQ>ZoeZ a{OñQ>a h¡& 1
1
(r ) (r ) (r )
1
1
2
 r and r can be any 8-bit or 16-bit General · r VWm r H$moB© ^r 8 {~Q> `m 16 {~Q> OZac a{OñQ>a
1
2
1
2
registers. Neither of them can be segment hmo gH$Vo h¢& CZ_| go H$moB© ^r goJ_oÝQ> a{OñQ>a Zht
register. hmo gH$Vm h¡&
 If the subtraction operation needs a bor- · `{X gãQ´>oŠeZ Am°naoeZ H$mo AÝV _| EH$ ~m°amo H$s
row at the end; CF (carry flag) is set. Thus, Amdí`H$Vm hmoVr h¡ Vmo CF (Ho$ar âcoJ) {ZYm©[aV
in subtraction operation CF works as a {H$`m OmVm h¡& Bg àH$ma, gãQ´>oŠeZ Am°naoeZ _| CF
Borrow Flag. This condition occurs when EH$ ~m°amo âcoJ Ho$ ê$n _| H$m`© H$aVm h¡& `h K{Q>V
r < r . hmoVm h¡ O~ r < r
2
1
2
1
 If r > r carry flag (CF) is reset because · `{X r > r Vmo Ho$ar âcoJ (CF) [agoQ> hmoVm h¡ Š`m|{H$
1
2
1
2
there is no borrow needed at the end. AÝV _| {H$gr ~m°amo H$s Amdí`H$Vm Zht hmoVr h¡&
If the result is negative CF is set and it is `{X n[aUm_ G$UmË_H$ h¡ Vmo CF goQ> {H$`m OmVm h¡
stored in 2's complement form.
VWm Bgo 2's H$åßcr_oÝQ> ê$n _| ñQ>moa {H$`m OmVm h¡&
SUB r, m and SUB m, r: Contents of SUB r, m and SUB m, r: S>mQ>m goJ_oÝQ> _| _o_moar
memory m in data segment and 8-bit or 16-bit m Ho$ H$ÝQ>oÝQ²>g VWm 8 {~Q> `m 16 {~Q> a{OñQ>g© Am°naoÝS²>g
registers are used as operands. Executed simi- Ho$ ê$n _| Cn`moJ {H$`o OmVo h¢& SUB r , r Ho$ g_mZ hr
lar to SUB r , r 1 2
1
2
EŠOrŠ`yQ> {H$`o OmVo h¢&
SUB r, data: Specified data is subtracted SUB r, data: {ZYm©[aV S>mQ>m H$mo a{OñQ>a Ho$ H$ÝQ>oÝQ²>g
from contents of register and the result is placed go KQ>m`m OmVm h¡ VWm n[aUm_ H$mo {ZYm©[aV a{OñQ>a _|
in the specified register. Executed similar to aIm OmVm h¡& SUB r , r Ho$ g_mZ hr EŠOrŠ`yQ> {H$`m
SUB r , r 1 2
2
1
OmVm h¡&
e.g. SUB AX, 4219H