Byte variable and buffer question

Don M

Lets say I have this:

var  byte  Inventory, Items[16]

Where Inventory contains the quantity in bits 7-4 and the item number in bits 3-0. The Items buffer just contains the quantities (from 00 to FF) of 16 different items designated by byte position 0 as item 1, byte position 1 as item 2, etc...

If I receive an Inventory byte value such as $13 which would represent a quantity of 1 (0001) of item number 3 (0011) how do I parse or breakout the quantity bits and the item number bits so that I can adjust the item number's quantity in the Items buffer?

I know that I would want to do something like this:

a := byte[@Items][?] (? = the item number from the item bits of Inventory) ' to get the quantity of ? item number
a := a + ? (? = the quantity bits from Inventory)  ' to add to the existing quantity number
byte[@Items][?] := a ' to store the new quantity number of ? item number

I don't know how to break down the byte into bits and get what I need.

Thanks.
Don

JonnyMac

qty  := byte[@Items][idx] >> 4
item := byte[@Items][idx] & $0F

...where idx is the index of the item you want to examine.

Don M

Thanks Jon.

Here is what I got out of your suggestion and it works the way I need it too.

qty  := Inventory >> 4    ' strips out quantity portion of byte
item := Inventory & $0F  ' strips out item number of byte

Thanks!
Don

Rayman

One trick is to use the INB, OUTB, or DIRB registers to access individual bits...

Don M

What I was looking for was to look at the 4 high bits and 4 low bits each as a group and figure out the hex (or decimal) value of each group.

Rayman

You can do things like:

INB:=items[x]
highbits:=INB[31..28]
lowbits:=INB[3..0]