LESSON4:

    If.. Then.. Else statements allow the program to branch off in many different directions. ASM does not support these staments directly, but using CP along with JR/JPyou can acheive the effect.

 

These are the instructions/ideas that will be covered in this lesson:

CP

IF... THEN...ELSE

 

CP
  CP simply sets the FLAG register (F) based on the subraction (A-s)

  • CP S  (where S = A,F,B,C,D,E,H,L,#,(hl),(iy+#),or (ix+#))

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 IF... THEN... ELSE
  To acheivean IF... THEN... ELSE statement(s) you must use a CP call followed by aJR, JP, or any other instruction that has conditionals (ret, call, ...) Here are some examples:

  ld b,5       ;b=5
  ld a,5       ;a=5
 
  cp b         ;set flags based on (a-b)
  jr z,AequalsB ;if a=b -> a-b=0,therefore
               ;if the Zero flag is set JUMP to AequalsB
  :
  .
AequalsB: ;
  :
  .

  You can also string them together like this:

  ld b,6  ;.
  ld c,5  ;.
  ld a,5  ;Set the init values
 
  cp b    ;IF a=b
  jr z,AB ;THEN goto AB
  cp c    ;IF a=c
  jr z,AC ;THEN goto AC
  jr NONE ;ELSE goto NONE (if it didn'tjump one of the other
         ; times then it must be an ELSE, NOTE:no conditia
         ; listed.  Just a straight 'jr NONE')
:
.
AB:
:
.
AC:
:
.
NONE:

  You've seen how to test for equality.  now for GREATER THAN, and LESS THAN:

  • Compare A and B for =, < and >
    LD B, 7
    LD A, 5


    CP B ; Flags = status(A-B)
    JP Z, A_Equal_To_B ; IF(a-b)=0 THEN a=b
    JP NC, A_Greater_Than_B ; IF(a-b)>0 THEN a>b
    JP C, A_Lower_Than_B ; IF(a-b)<0 THEN a<b


    A_Greater_Than_B:
    (...)
    JP end_compare


    A_Lower_Than_B:
    (...)
    JP end_compare


    A_Equal_To_B:
    (...)


    end_compare:


    RET
    END

:
:
.

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LESSON3     INDEX     LESSON5

This is the end of the lesson, I do not guarantee any correctness of any statements made above.