NAME	DBcc -- Decrement and branch conditionally
SYNOPSIS	DBcc Dn,<label> Size of offset = (Word)
FUNCTION	Controls a loop of instructions. The parameters are: a condition code, a data register (counter), and an offset value. The instruction first tests the condition (for termination); if it is true, no operation is performed. If the termination condition is not true, the low-order 16 bits of the counter are decremented by one. If the result is -1, execution continues at the next instruction, otherwise, execution continues at the specified address. Condition code 'cc' specifies one of the following: 0000 F False Z = 1 0001 T True Z = 0 0010 HI HIgh C + Z = 0 0011 LS Low or Same C + Z = 1 0100 CC Carry Clear C = 0 0101 CS Carry Set C = 1 0110 NE Not Equal Z = 0 0111 EQ EQual Z = 1 1000 VC oVerflow Clear V = 0 1001 VS oVerflow Set V = 1 1010 PL PLus N = 0 1011 MI MInus N = 1 1100 GE Greater or Equal N (+) V = 0 1101 LT Less Than N (+) V = 1 1110 GT Greater Than Z + (N (+) V) = 0 1111 LE Less or Equal Z + (N (+) V) = 1 Keep the following in mind when using DBcc instructions: A DBcc acts as the UNTIL loop contruct in high level languages. E.g., DBMI would be "decrement and branch until minus". Most assemblers accept DBRA or DBF for use when no condition is required for termination of a loop. The DBcc will, unlike the Bcc instruction, take the branch only if the set of conditions are NOT satified. The loop will be terminated if the condition is true, or the counter is zero BEFORE a decrement, and wrap to -1. This mean that if you execute code like:   move.w #30,d0       .Loop       move.l (a0)+,(a1)+   dbf d0,.Loop         then the copy will be executed *31* times, and 124 bytes of memory will be copied, not 120.         A good practice is therefore to write:         move.w #31-1,d0       .Loop       move.l (a0)+,(a1)+   dbf d0,.Loop         To compare two strings that may be in excess of 64k length for beeing equal, you could execute the following code:   ...         move.l #$53452-1,d0   beq.s .NoLength ; Zero length!   bra.s .StartOuterLoop; The upper word contain count of 65536's...       .OuterLoop       swap d0       .InnerLoop      cmp.b (a0)+,(a1)+  dbne d0,.InnerLoop; Remember, drop trough on condition TRUE.       .StartOuterLoop  ; d0 will be $FFFF on 2.+ run-through   bne.s .NotEqual; Dropped through due to Not Equal?   swap d0; Get upper part of word...   dbf d0,.OuterLoop   ...         It would not be possible to use two sets of DBNEs, as SWAP changes the condition codes - and we don't want the drop- though to be on account of D0, instead of the compares...         Another neat trick is to use instruction as a conditional decrementer; this code will decrement d0.w if the last instruction returned NOT EQUAL:   ...   dbeq d0,.Next       .Next       ...
FORMAT	15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 0 1 CONDITION 1 1 0 0 1 REGISTER 16 BITS OFFSET (d16) "CONDITION" is one of the condition code given some lines before. "REGISTER" is the number of data register. Offset is the relative gap in byte to do branching.
RESULT	Not affected.

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