Apple II User Manual

     from the memory location whose address now resides in Rn. Rn is

     again decremented by 1 and the low-order ACC byte is loaded from

     the corresponding memory location. Branch conditions reflect the

     final ACC contents. The carry is cleared. Because Rn is

     decremented prior to loading each of the ACC halves, double-byte

     stacks may be implemented with the STD @Rn and POPD @Rn ops

     (Rn is the stack pointer).

     EXAMPLE:

     15 34 A0   SET  R5   $A034     ;Init stack pointer

     10 12 AA   SET  R0   $AA12     ;Load $AA12 into ACC

     75         STD  @R5            ;Push $AA12 onto stack

     10 34 BB   SET  R0   $BB34     ;Load $BB34 into ACC

     75         STD  @R5            ;Push $BB34 onto stack

     C5         POPD @R5            ;Pop $BB34 off stack

     C5         POPD @R5            ;Pop $AA12 off stack

COMPARE:

     CPR Rn              [ Dn ]

     The ACC (R0) contents are compared to Rn by performing the 16

     bit binary subtraction ACC-Rn and storing the low order 16

     difference bits in R13 for subsequent branch tests. If the 16

     bit unsigned ACC contents are greater than or equal to the 16

     bit unsigned Rn contents, then the carry is set, otherwise it

     is cleared. No other registers, including ACC and Rn, are

     disturbed.

     EXAMPLE:

     15 34 A0           SET  R5   $A034     ;Pointer to memory

     16 BF A0           SET  R6   $A0BF     ;Limit address

     B0         LOOP1   SUB  R0             ;Zero data

     75                 STD  @R5            ;clear 2 locations

                                            ;increment R5 by 2

     25                 LD   R5             ;Compare pointer R5

     D6                 CPR  R6             ;to limit R6

     02 FA              BNC  LOOP1          ;loop if C clear

INCREMENT:

     INR Rn              [ En ]

     The contents of Rn are incremented by 1. The carry is cleared

     and other branch conditions reflect the incremented value.

     EXAMPLE:

     15 34 A0   SET  R5   $A034     ;(Pointer)

     B0         SUB  R0             ;Zero to R0

     55         ST   @R5            ;Clr Location $A034

     E5         INR  R5             ;Incr R5 to $A036

     55         ST   @R5            ;Clrs location $A036

                                    ;(not $A035)