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Maxim Integrated 19-13

MAX31782 User’s Guide

Revision 0; 8/11

19.7.5 Looping Operations

Looping over a section of code can be performed by using the conditional jump instructions. However, there is built-in

functionality, in the form of the ‘DJNZ LC[n], src’ instruction, to support faster, more compact looping code with separate

loop counters. The 16-bit registers LC[0], and LC[1] are used to store these loop counts. The ‘DJNZ LC[n], src’ instruc-

tion automatically decrements the associated loop counter register and jumps to the loop address specified by src if

the loop counter has not reached 0.

To initialize a loop, set the LC[n] register to the count you wish to use before entering the loop’s main body.

The desired loop address should be supplied in the src operand of the ‘DJNZ LC[n], src’ instruction. When the sup-

plied loop address is relative (+127/-128 words) to the DJNZ LC[n] instruction, as is typically the case, the assembler

automatically calculates the relative offset and inserts this immediate value in the object code.

move LC[1],#10h  ;loop16times

LoopTop:    ;loopaddrrelativetodjnzLC[n],srcinstruction

call LoopSub

djnz LC[1],LoopTop ;decrementLC[1]andjumpifnonzero

When the supplied loop address is outside the relative jump range, the prefix register (PFX[0]) is used to supply the

high byte of the loop address as required.

move LC[1],#10h  ;loop16times

LoopTop:    ;loopaddrnotrelativetodjnzLC[n],src

call LoopSub

...

djnz LC[1],LoopTop ;decrementLC[1]andjumpifnonzero

     ;assemblesto: movePFX[0],#high(LoopTop)

     ;   djnzLC[1], #low(LoopTop)

If loop execution speed is critical and a relative jump cannot be used, one might consider preloading an internal 16-bit

register with the src loop address for the ‘DJNZ LC[n], src’ loop. This ensures that the prefix register will not be needed

to supply the loop address and always yields the fastest execution of the DJNZ instruction.

move LC[0],#LoopTop ;usingLC[0]asaddressholdingregister

     ;assemblesto: movePFX[0],#high(LoopTop)

     ;   moveLC[0], #low(LoopTop)

move LC[1],#10h  ; loop16times

...

LoopTop:    ;loopaddressnotrelativetodjnzLC[n],src

call LoopSub

...

djnz LC[1],LC[0] ;decrementLC[1]andjumpifnonzero

If opting to preload the loop address to an internal 16-bit register, the most time and code efficient means is by perform-

ing the load in the instruction just prior to the top of the loop:

move LC[1],#10h  ;Setloopcounterto16

move LC[0],IP  ;Setloopaddresstothenextaddress

LoopTop:    ;loopaddrnotrelativetodjnzLC[n],src

...

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