Hardware division

alan · Joined: 12 Nov 2012 Posts: 357 Location: South Africa

Good day,

How would I code the following in assembly?

Ttelmah · Joined: 11 Mar 2010 Posts: 19554

You can't.
A float number has no concept of decimal places. In fact it won't even guarantee to hold any number accurately. 33000.1 for example will actually be stored internally as 33000.102 since this is the closest representation possible.

Just store integer 'tenths'. So /1000, so you have 122. Then the %LW output will print this as if it has one decimal.

122 will print as 12.2 etc..

alan · Joined: 12 Nov 2012 Posts: 357 Location: South Africa

Hi Ttelmah,

Everything is already in integer. I just need to scale by 10000.
So something like 123456 will become 12. this i send through to someone else who will display as 1.2 for instance.

I don't want float and have never used it before.
I want to use the div.sd instruction, but my assembler are not that good, can understand when I read the listing but that is as far as it go. Last time I used assembler with MPLAB was in 1999.

Regards

jeremiah · Joined: 20 Jul 2010 Posts: 1358

Try using the ldiv() function and look at the assembly it generates. I might be misunderstanding what you are looking for though.

alan · Joined: 12 Nov 2012 Posts: 357 Location: South Africa

I'll try again.

I want something similar to the _mul() function

jeremiah · Joined: 20 Jul 2010 Posts: 1358

did you try the ldiv() function?

I don't know off the top of my head if it implements the same or not. It might be different.

The repeat instruction is required for hardware division to work.

EDIT: Here is the relevant blurb from the datasheet:

alan · Joined: 12 Nov 2012 Posts: 357 Location: South Africa

Sorry I should have included the asm in previous reply.

Ttelmah · Joined: 11 Mar 2010 Posts: 19554

A standard integer division does what he is describing.
The point about ldiv, is it gives you access to the remainder, which he is not talking about.

Key is that there is no equivalent to mul. A numeric multiplication can overflow. The point about mul is you start with two smaller numbers (8 bit say), and keep the overflow to give you a 16bit result. On a division, you need to be using the larger maths type to actually perform the division. You can simply just take the smaller result when finished.

Declare a structure containing two int16's 'MSW & LSW'. Then declare a union between this and the int32. Simply perform a standard division by 10000, and then access the LSW as the result. No casting needed, and you are now using the smaller type.