DSPIC30F datasheet(29/248 Pages) MICROCHIP

29 / 248 page

 2004 Microchip Technology Inc.

Preliminary

DS70083G-page 27

dsPIC30F

2.4

Divide Support

The dsPIC devices feature a 16/16-bit signed fractional

divide operation, as well as 32/16-bit and 16/16-bit

signed and unsigned integer divide operations, in the

form of single instruction iterative divides. The following

instructions and data sizes are supported:

DIVF

- 16/16 signed fractional divide

DIV.sd

- 32/16 signed divide

DIV.ud

- 32/16 unsigned divide

DIV.sw

- 16/16 signed divide

DIV.uw

- 16/16 unsigned divide

The 16/16 divides are similar to the 32/16 (same number

of iterations), but the dividend is either zero-extended or

sign-extended during the first iteration.

The quotient for all divide instructions is stored in W0,

and the remainder in W1. DIV and DIVF can specify

any W register for both the 16-bit dividend and divisor.

All other divides can specify any W register for the

16-bit divisor, but the 32-bit dividend must be in an

aligned W register pair, such as W1:W0, W3:W2, etc.

The non-restoring divide algorithm requires one cycle

for an initial dividend shift (for integer divides only), one

cycle per divisor bit, and a remainder/quotient correc-

tion cycle. The correction cycle is the last cycle of the

iteration loop but must be performed (even if the

remainder is not required) because it may also adjust

the quotient. A consequence of this is that DIVF will

also produce a valid remainder (though it is of little use

in fractional arithmetic).

The divide instructions must be executed within a

REPEAT loop. Any other form of execution (e.g., a

series of discrete divide instructions) will not function

correctly because the instruction flow depends on

RCOUNT. The divide instruction does not automatically

set up the RCOUNT value and it must, therefore, be

explicitly and correctly specified in the REPEAT instruc-

tion as shown in Table 2-1 (REPEAT will execute the tar-

get instruction {operand value+1} times). The REPEAT

loop count must be setup for 18 iterations of the DIV/

DIVF

instruction. Thus, a complete divide operation

requires 19 cycles.

TABLE 2-1:

DIVIDE INSTRUCTIONS

Note:

The divide flow is interruptible. However,

the user needs to save the context as

appropriate.

Instruction

Function

DIVF

Signed fractional divide: Wm/Wn

→ W0; Rem → W1

DIV.sd

Signed divide: (Wm+1:Wm)/Wn

→ W0; Rem → W1

DIV.sw or

DIV.s

Signed divide: Wm/Wn

→ W0; Rem → W1

DIV.ud

Unsigned divide: (Wm+1:Wm)/Wn

→ W0; Rem → W1

DIV.uw or

DIV.u

Unsigned divide: Wm/Wn

→ W0; Rem → W1

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