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ST9 USER GUIDE

This “eight bits signed by eight bits unsigned” multiplication with a “sixteen bits signed” result

takes a maximum of 36 cycles.

There are two divide instructions.

The div instruction divides a word by a byte, and returns the quotient and the remainder as the

low and the high bytes of the destination respectively. For example:

ldw

rr0,#31184

; rr0=#31184

r2,#201

; r2=#201

div

rr0,r2

; rr0=1D9Bh, 1Dh=#29 and 9Bh=#155

This puts the value 155 in r1 (the quotient) and the value 29 (the remainder) in r0, and r2 still

contains 201.

If the divider is greater than the dividend, nothing is done. If the divisor is zero, a trap is trig-

gered that acts like an interrupt request, and uses the vector at locations 2 and 3 in program

memory. It is up to you to write the appropriate code to handle this trap. Finally, the numbers

to be divided should be such that the quotient be less than 256, that is, can be stored in a

single byte. Otherwise, the results are undefined.

The usable result is only the data stored in r1 which is 155 (for the previous example), the re-

mainder must be divided by the divisor (201) to give more precision (16-bits precision).

ldw

rr0,#31184

; rr0=#31184

r2,#201

; r2=#201

div

rr0,r2

; rr0=1D9Bh, 1Dh=#29 the remainder and

; 9Bh=#155 the quotient

r4,r1

; r4=9Bh=#155

clr

; rr0=1D00h

div

rr0,r2

; rr0=0BC24h, 0BCh=#188 the remainder and

; 24h=#36 the quotient

r0,r4

; rr0=09B24h, 09B24h means in fix-point

; with the point in the 16-bits middle

; 09B24h=155.140625 instead of

; #31184/#201=155.1442786

In the best case, the number of cycles required to divide a word by a byte with 16-bits preci-

sion is 80 cycles. This program has to manage overflow and divide by zero functions in order

to be able to be used.

The divws instruction performs one of the sixteen partial divides required to divide a double

word by a word, so you need to write a subroutine to perform the division completely. An ex-

ample subroutine is given in the ST9 Programming Manual.

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