Implementing an 8 bit fixed point register

**moogyd@yahoo.co.uk** · Jul 1 '08, 11:05 AM

Re: Implementing an 8 bit fixed point register

On 1 Jul, 08:57, nickooooola <nick83...@gmai l.comwrote:

Hello to all
I'm about to write a simulator for a microcontroller in python
(why python? because I love it!!!)
>
but I have a problem.
>
The registry of this processor are all 8 bit long (and 10 bit for some
other strange register)
and I need to simulate the fixed point behaviour of the register,
and to access the single bit.
>
f.x. (this is a pseudo python session, only for understanding)
>

>reg1 = fixed_int(8)
>reg2 = fixed_int(10)
>reg1[0].set()

or

>reg1[0] = 1 # or True? how to rapresent a binary bit
>reg1[0]

1

>reg1[1]

0

>reg1[9]

>
<exception .... blah blah not in range>>>reg2 = 0x7FE # in binary 11111111110 , or 11 bit long

>reg2

>
0x7FE
#or 1111111110, the memorization truncate the upper bits ( or perhaps
generate an exception?)>>re g2 += 0x02 #the result is 10000000000, again not contained in 10 bit

>reg2

>
0x00
# truncated again>>myproces sor.flags['z']
>
1
# or True? Z flag indicated an overflow in arithmetic operations
>
Is possibile to do so in python?
>
thanks

I am not sure if it is exactly what you are looking for (it may be a
bit low level), but myHDL may be interesting. It provides low level H/
W simulation capabilities.

http://myhdl.jandecaluwe.com/doku.php

Steven

**Mel** · Jul 1 '08, 12:55 PM

Re: Implementing an 8 bit fixed point register

nickooooola wrote:

Hello to all
I'm about to write a simulator for a microcontroller in python
(why python? because I love it!!!)

[...]

The registry of this processor are all 8 bit long (and 10 bit for some
other strange register)
and I need to simulate the fixed point behaviour of the register,
and to access the single bit.
>
f.x. (this is a pseudo python session, only for understanding)
>

>>>reg1 = fixed_int(8)
>>>reg2 = fixed_int(10)
>>>reg1[0].set()

or

>>>reg1[0] = 1 # or True? how to rapresent a binary bit
>>>reg1[0]

1

>>>reg1[1]

0

>>>reg1[9]

>>>reg2 = 0x7FE # in binary 11111111110 , or 11 bit long
>>>reg2

0x7FE
#or 1111111110, the memorization truncate the upper bits ( or perhaps
generate an exception?)

>>>reg2 += 0x02 #the result is 10000000000, again not contained in 10 bit
>>>reg2

0x00
# truncated again

>>>myprocessor. flags['z']

1
# or True? Z flag indicated an overflow in arithmetic operations
>
Is possibile to do so in python?

I did this for a PIC, and generally just used brute force, e.g.

temp = reg2 + 0x02
reg2 = temp & 0xFF
if temp & 0x100:
flags |= CARRY_MASK
else:
flags &= ~CARRY_MASK
if temp & 0xFF:
flags &= ~ZERO_MASK
else:
flags |= ZERO_MASK

Since it was a PIC, and there were only a half-dozen arithmetic/logical
operations, the code stayed in this form. With something bigger, it would
have been attractive to wrap these operations up in a class, as you
suggest.. (just a sketch) ..

class fixed_register (object):
def __init__ (self, size, val=0):
self.mask = ~(-1 << size)
self.ovfmask = 1 << size
self.val = val & self.mask

with appropriate __add__, __sub__, etc. Perhaps __getitem__, __setitem__ to
manipulate bits.

Good Luck, Mel.

**Grant Edwards** · Jul 1 '08, 02:15 PM

Re: Implementing an 8 bit fixed point register

On 2008-07-01, nickooooola <nick83ola@gmai l.comwrote:

Hello to all
I'm about to write a simulator for a microcontroller in python
(why python? because I love it!!!)
>
but I have a problem.
>
The registry of this processor are all 8 bit long (and 10 bit
for some other strange register) and I need to simulate the
fixed point behaviour of the register,

Somebody posted a class that impliments fixed-width integer
types a while back.

and to access the single bit.

The bitwise operators &, |, ~, ^ all work just like they do in C.

You can write methods to dip those in syntactic sugar if you
want.

f.x. (this is a pseudo python session, only for understanding)
>

>>>reg1 = fixed_int(8)
>>>reg2 = fixed_int(10)
>>>reg1[0].set()

or

>>>reg1[0] = 1 # or True? how to rapresent a binary bit
>>>reg1[0]

1

>>>reg1[1]

0

>>>reg1[9]

><exception .... blah blah not in range>

>>>reg2 = 0x7FE # in binary 11111111110 , or 11 bit long
>>>reg2

0x7FE
#or 1111111110, the memorization truncate the upper bits ( or perhaps
generate an exception?)

>>>reg2 += 0x02 #the result is 10000000000, again not contained in 10 bit
>>>reg2

0x00
# truncated again

>>>myprocessor. flags['z']

1
# or True? Z flag indicated an overflow in arithmetic operations
>
Is possibile to do so in python?

Yes. Everything shown above is possible. If you really want
to get clever, you'll want to read up on the __setattr__,
__getattr__, __getitem__, and __setitem__ object methods.

They'll allow you to define special handling for the semantics
below:

foo.something = x # calls foo.__setattr__ _('something',x )
x = foo.something # calls foo.__getattr__ _('something,)

foo[n] = x # calls foo.__setitem__ (n,x)
x = foo[n] # calls foo.__getitme__ (n)

In the latter two cases, you can support slicing if you want.
That could allows you to grab a "bitfield" out of a register:

x = processor.regA[4:7] # get bits 4,5,6

processor.regA[4:7] # set bits 4,5,6

Just remember that in Python slices are traditionally half-open
intervals -- they don't include the "right" endpoint. That's
going to confuse people who are more used to reading processor
data sheets where bit-ranges are traditionally closed
intervals. You _could_ implment your __[sg]etitem__ slice
handling so that they're treated as closed intervals. That will
be a lot more intuitive to people used to dealing with
microprocessor register definitions, but could confuse an
experienced Python programmer.

Writing methods for __setattr__ and __getattr__ is a little
tricky. It's very easy to end up with infinite recursion. It's
not that hard to fix/avoid, but it takes a while to get the
hang of it.

--
Grant Edwards grante Yow!
at
visi.com

**nickooooola** · Jul 1 '08, 03:35 PM

Re: Implementing an 8 bit fixed point register

Thanks to all for the responses!

to MEl: I also want to build a pic simulator, but I want to do this as
"python for big project" learning exercise, I have used python in the
past only for small script and now I want to use it for something more
"big".
do you have some code to share?

myhdl seems interesting, I think that I can take a blick on its source
code...I think that must have something like
the 8 bit register class that I need somewhere

I try what you people have said, and if it turns to be something
useful, I report it to the community.

Nicola

Ps
excuse me for my english...

**Terry Reedy** · Jul 1 '08, 07:15 PM

Re: Implementing an 8 bit fixed point register

nickooooola wrote:

Hello to all
I'm about to write a simulator for a microcontroller in python
(why python? because I love it!!!)
>
but I have a problem.
>
The registry of this processor are all 8 bit long (and 10 bit for some
other strange register)
and I need to simulate the fixed point behaviour of the register,
and to access the single bit.

In Python3, I would use a (mutable) bytearray.

IDLE 3.0b1

>>reg1 = bytearray((0,)* 8) # or *10 for 10 bits
>>reg1

bytearray(b'\x0 0\x00\x00\x00\x 00\x00\x00\x00' )

>>reg1[1]=1
>>reg1[1]

1

>>tuple(reg1)

(0, 1, 0, 0, 0, 0, 0, 0)

A bytearray subclass could enforce that all 'bits' (stored as bytes) are
0 or 1, have a customized representation to your taste, and add methods
like .flipall().

The overhead of using 8 bytes instead of 1 to hold the object value is
actually small compared to the minimum object size of 16 bytes (on Win32XP).

>>sys.getsizeof (reg1)

24

In Python2.x, you can use the array module to make equivalent mutable
arrays of chars.

Terry Jan Reedy

**Grant Edwards** · Jul 1 '08, 07:45 PM

Re: Implementing an 8 bit fixed point register

On 2008-07-01, Terry Reedy <tjreedy@udel.e duwrote:

>
>
nickooooola wrote:

>Hello to all
>I'm about to write a simulator for a microcontroller in python
>(why python? because I love it!!!)
>>
>but I have a problem.
>>
>The registry of this processor are all 8 bit long (and 10 bit for some
>other strange register)
>and I need to simulate the fixed point behaviour of the register,
>and to access the single bit.

>
In Python3, I would use a (mutable) bytearray.
>
IDLE 3.0b1

>reg1 = bytearray((0,)* 8) # or *10 for 10 bits
>reg1

bytearray(b'\x0 0\x00\x00\x00\x 00\x00\x00\x00' )

>reg1[1]=1
>reg1[1]

1

>tuple(reg1)

(0, 1, 0, 0, 0, 0, 0, 0)
>
A bytearray subclass could enforce that all 'bits' (stored as bytes) are
0 or 1, have a customized representation to your taste, and add methods
like .flipall().

It seems like implementing ALU operations on such arrays would
be a lot more work than implementing bit-indexing on a type
derived on a more "integer" like base. I'm pretty fuzzy on how
one sub-classes basic things like integers, so maybe I'm all
wet, and adding __getitem__ and __setitem__ to an integer type
isn't even possible.

--
Grant Edwards grante Yow! Do you guys know we
at just passed thru a BLACK
visi.com HOLE in space?

**Terry Reedy** · Jul 1 '08, 10:15 PM

Re: Implementing an 8 bit fixed point register

Grant Edwards wrote:

On 2008-07-01, Terry Reedy <tjreedy@udel.e duwrote:

>>
>nickooooola wrote:

>>Hello to all
>>I'm about to write a simulator for a microcontroller in python
>>(why python? because I love it!!!)
>>>
>>but I have a problem.
>>>
>>The registry of this processor are all 8 bit long (and 10 bit for some
>>other strange register)
>>and I need to simulate the fixed point behaviour of the register,
>>and to access the single bit.

>In Python3, I would use a (mutable) bytearray.
>>
>IDLE 3.0b1

>>>>reg1 = bytearray((0,)* 8) # or *10 for 10 bits
>>>>reg1

>bytearray(b'\x 00\x00\x00\x00\ x00\x00\x00\x00 ')

>>>>reg1[1]=1
>>>>reg1[1]

>1

>>>>tuple(reg 1)

>(0, 1, 0, 0, 0, 0, 0, 0)
>>
>A bytearray subclass could enforce that all 'bits' (stored as bytes) are
>0 or 1, have a customized representation to your taste, and add methods
>like .flipall().

>
It seems like implementing ALU operations on such arrays would
be a lot more work than implementing bit-indexing on a type
derived on a more "integer" like base. I'm pretty fuzzy on how
one sub-classes basic things like integers, so maybe I'm all
wet, and adding __getitem__ and __setitem__ to an integer type
isn't even possible.

If one only wants bit operations, then the array approach is easy. If
one only wants int arithmetic and all-bits logic, then int approach is
easy. OP did not completely specify needs.

The problem with the int approach is that ints are immutable.
Therefore, if one wants to subclass int to hide the bit masking for the
bit operations, one must override *every* operation one might use,
including all arithmetic and all-bits logic, even when the int operation
gives the correct answer other than the class of the result.

class bits(int):
....
def __add__(self,ot her):
return bit(self+other)
....

If one does not,

a,b = bits(1),bits(2)
c = a+b #c is now an int, not a bits

So there is a tendency to not subclass and instead either leave the
extra functionality unmasked in repeated code or to put it in functions
instead.

setters = (1,2,4,8,16,32, 64, ..., 2147483648)# slightly pseudocode
unsetters = (~1,~2,~4,...~2 147483648) # ditto
def bitset(n, bit): return n | setters[bit]
def bitunset(n,bit) : return n & unsetters[bit]

thus not getting the nice reg[n] functionality, nor an easy display of
individual bits without callings *another* function.

One the other hand, with mutable arrays, setting bits is a mutation and
so no override of __setitem__ is required unless one wants to be fancy
and enforce setting to 0 or 1.

It is a trade-off.

Terry Jan Reedy

**Grant Edwards** · Jul 1 '08, 10:45 PM

Re: Implementing an 8 bit fixed point register

On 2008-07-01, Terry Reedy <tjreedy@udel.e duwrote:

>>A bytearray subclass could enforce that all 'bits' (stored as
>>bytes) are 0 or 1, have a customized representation to your
>>taste, and add methods like .flipall().

>>
>It seems like implementing ALU operations on such arrays would
>be a lot more work than implementing bit-indexing on a type
>derived on a more "integer" like base. I'm pretty fuzzy on how
>one sub-classes basic things like integers, so maybe I'm all
>wet, and adding __getitem__ and __setitem__ to an integer type
>isn't even possible.

>
If one only wants bit operations, then the array approach is
easy. If one only wants int arithmetic and all-bits logic,
then int approach is easy. OP did not completely specify
needs.

He said he's writing a microprocessor simulator, so he's going
to want integer operations, all-bits logical operations, and
individual bit acess (by number and by name) and bit-slice
access.

The problem with the int approach is that ints are immutable.

That dawned on me after I started googling around a little.

Therefore, if one wants to subclass int to hide the bit
masking for the bit operations, one must override *every*
operation one might use, including all arithmetic and all-bits
logic, even when the int operation gives the correct answer
other than the class of the result.

Since we're doing fixed-width operations, Python's int
operations don't give the correct answer other than the class
of the result.

class bits(int):
...
def __add__(self,ot her):
return bit(self+other)
...
>
If one does not,
>
a,b = bits(1),bits(2)
c = a+b #c is now an int, not a bits
>
So there is a tendency to not subclass and instead either leave the
extra functionality unmasked in repeated code or to put it in functions
instead.
>
setters = (1,2,4,8,16,32, 64, ..., 2147483648)# slightly pseudocode
unsetters = (~1,~2,~4,...~2 147483648) # ditto
def bitset(n, bit): return n | setters[bit]
def bitunset(n,bit) : return n & unsetters[bit]

On a machine with a barrel shifter, it's probably faster to do
this:

def bitset(n,bit): return n | (1<<bit)
def bitclr(n,bit): return n & ~(1<<bit)

But, your approach could be easily modified to support slices:

import operator
masks = [(1<<n) for n in range(32)]
def mask(bits):
if type(bits) is slice:
return reduce(operator .__or__,masks[bits])
return masks[bits]

def bitset(n,bits):
return n | mask(bits)

Is there a literal syntax for a slice? This doesn't seem to
work:

bitset(n,0:4)

thus not getting the nice reg[n] functionality, nor an easy
display of individual bits without callings *another*
function.
>
One the other hand, with mutable arrays, setting bits is a
mutation and so no override of __setitem__ is required unless
one wants to be fancy and enforce setting to 0 or 1.

More importantly, I presume that slices are supported so when
you need values of bit-fields, you can do this:

op = instruction[6:8]
src = instruction[3:6]
dest = instruction[0:3]

The half-closed interval notation for slices is probably going
to drive the programmer up the wall because all of the
documentation that's being followed uses closed intervals.

It is a trade-off.

And Python programmers are awfully spoiled. :)

--
Grant Edwards grante Yow! I think my career
at is ruined!
visi.com

Implementing an 8 bit fixed point register

Implementing an 8 bit fixed point register

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