DECREMENT:
DCR Rn [ Fn ]
The contents of Rn are decremented by 1. The carry is cleared
and other branch conditions reflect the decremented value.
EXAMPLE: (Clear 9 bytes beginning at location A034)
15 34 A0 SET R5 $A034 ;Init pointer
14 09 00 SET R4 9 ;Init counter
B0 SUB R0 ;Zero ACC
55 LOOP2 ST @R5 ;Clear a mem byte
F4 DCR R4 ;Decrement count
07 FC BNZ LOOP2 ;Loop until Zero
Non-Register Instructions:
--------------------------
RETURN TO 6502 MODE:
RTN 00
Control is returned to the 6502 and program execution continues
at the location immediately following the RTN instruction. the
6502 registers and status conditions are restored to their
original contents (prior to entering SWEET 16 mode).
BRANCH ALWAYS:
BR ea [ 01 d ]
An effective address (ea) is calculated by adding the signed
displacement byte (d) to the PC. The PC contains the address
of the instruction immediately following the BR, or the address
of the BR op plus 2. The displacement is a signed two's
complement value from -128 to +127. Branch conditions are not
changed.
NOTE: The effective address calculation is identical to that
for 6502 relative branches. The Hex add & Subtract features of
the APPLE ][ monitor may be used to calculate displacements.
d = $80 ea = PC + 2 - 128
d = $81 ea = PC + 2 - 127
d = $FF ea = PC + 2 - 1
d = $00 ea = PC + 2 + 0
d = $01 ea = PC + 2 + 1
d = $7E ea = PC + 2 + 126
d = $7F ea = PC + 2 + 127
EXAMPLE: