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dbcc.md

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DBcc - Test condition, decrement, and branch

Operation

IF(condition false)
  THEN [Dn] ← [Dn] - 1 {decrement loop counter}
    IF [Dn] = -1
      THEN [PC] ← [PC] + 2 {fall through to next instruction}
      ELSE [PC] ← [PC] + d {take branch}
  ELSE [PC] ← [PC] + 2 {fall through to next instruction}

Syntax

DBcc Dn,<label>

Attributes

Size word

Description

The DBcc instruction provides an automatic looping facility and replaces the usual decrement counter, test, and branch instructions. Three parameters are required by the DBcc instruction: a branch condition (specified by 'cc'), a data register that serves as the loop down-counter, and a label that indicates the start of the loop. The DBcc first tests the condition 'cc', and if 'cc' is true the loop is terminated and the branch back to <label> not taken. The 14 branch conditions supported by Bcc are also supported by DBcc, as well as DBF and DBT (F = false, and T = true). Note that many assemblers permit the mnemonic DBF to be expressed as DBRA (i.e., decrement and branch back).

It is important to appreciate that the condition tested by the DBcc instruction works in the opposite sense to a Bcc, conditional branch, instruction. For example, BCC means branch on carry clear, whereas DBcc means continue (i.e., exit the loop) on carry clear. That is, the DBcc condition is a loop terminator. If the termination condition is not true, the low-order 16 bits of the specified data register are decremented. If the result is -1, the loop is not taken and the next instruction is executed. If the result is not -1, a branch is made to 'label'. Note that the label represents a 16-bit signed value, permitting a branch range of -32K to +32K bytes. Since the value in Dn decremented is 16 bits, the loop may be executed up to 64K times.

We can use the instruction DBEQ, decrement and branch on zero, to mechanize the high-level language construct REPEAT...UNTIL.

LOOP ...            REPEAT
     ...
     ...              [D0] := [D0] - 1
     ...
     DBEQ D0,REPEAT UNTIL [D0] = - 1 OR [Z] = 1

Application

Suppose we wish to input a block of 512 bytes of data (the data is returned in register D1). If the input routine returns a value zero in D1, an error has occurred and the loop must be exited.

      LEA     Dest,A0   ;Set up pointer to destination
      MOVE.W  #511,D0   ;512 bytes to be input
AGAIN BSR     INPUT     ;Get the data in D1
      MOVE.B  D1,(A0)+  ;Store it
      DBEQ    D0,AGAIN  ;REPEAT until D1 = 0 OR 512 times

Condition codes

X N Z V C
- - - - -

Not affected

From MOTOROLA M68000 FAMILY Programmer's reference manual. Copyright 1992 by Motorola Inc./NXP. Adapted with permission.