roy@gitpyr.UUCP (Roy J. Mongiovi) (12/18/84)
I'm sort of curious about the possible reasons behind a little quirk
in the way the 8086 family of processors sets the status bits. What
is the value of having the INC and DEC instructions set all the bits
EXCEPT for the carry flag? I mean, there must be some reason, right?
Is it really useful to be able to do a comparison followed by an INC
(or DEC), and then be able to do a JC or JNC on the comparison?
--
Roy J. Mongiovi. Office of Computing Services. User Services.
Georgia Institute of Technology. Atlanta GA 30332. (404) 894-6163
...!{akgua, allegra, amd, hplabs, ihnp4, masscomp, ut-ngp}!gatech!gitpyr!roy
Who me? I'm not even a REAL modo, I'm only a quasi-modo.baba@spar.UUCP (12/19/84)
> > Is it really useful to be able to do a comparison followed by an INC > (or DEC), and then be able to do a JC or JNC on the comparison? > -- It makes the equivalent of the C "while(*pointer++ > VALUE)" construct more efficient. In preserving only the carry flag it would appear that intel learned from their mistake on the 8080, where 16-bit increments and decrements don't even affect the zero flag, forcing two additional instructions into every 16-bit down-counter.
tim@cmu-cs-k.ARPA (Tim Maroney) (12/21/84)
Of course it is useful to set/clear the Z status bit in the 8080 on a DEC instruction! Loops that count down to zero are much easier. Exemplia gratia: mvi b,limit loop: ; whatever the loop does dec b jnz loop I wish they had done the same with DCX instructions, so you could count down using 16-bit integers, but no, you have to go: lxi b,limit16 loop: ; whatever dcx b mov a,b ora c jnz loop which is grodier to the nth degree. -=- Tim Maroney, Carnegie-Mellon University Computation Center ARPA: Tim.Maroney@CMU-CS-K uucp: seismo!cmu-cs-k!tim CompuServe: 74176,1360 audio: shout "Hey, Tim!" "Remember all ye that existence is pure joy; that all the sorrows are but as shadows; they pass & are done; but there is that which remains." Liber AL, II:9.
wapd@houxj.UUCP (Bill Dietrich) (12/22/84)
I haven't used an 8080-type processor in 8 years or so, but I believe the reason for INC/DEC not touching CARRY is to allow multiple-digit or high-precision math. To add two 200 digit numbers, for example (in pseudo-code) : p1 = pointer to digit of number 1 (source) p2 = pointer to digit of number 2 (source) p3 = pointer to digit of number 3 (destination) *px means digit pointed to by px counter = 200 loop: add with carry *p1 and *p2 putting in *p3, generating carry INC p1 INC p2 INC p3 DEC counter JUMP to loop if counter not zero Bill Dietrich houxj!wapd