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panky

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Joined: 02/10/2012
Location: Australia
Posts: 1116

Posted: 05:39am 28 Feb 2018

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I am working on an emeulation program for an old 8 bit computer and needed to simulate the memory. I need to read and write 8 bits of data at a time with an indexing pointer (the Program Counter in old 8 bit machines).

I looked at Long Strings but decided I could do it with an integer array and developed the two routines below to simulate reading from and writing to an arbitrary length memory stack from address 0 to whatever maximum is desired within the constraints of the MMs available memory.

I hope this may be of use to anyone doing anything similar.

Doug.

Quote
'*******************************************************
' A couple of routines to simulate the getting and putting
' of 8 bits of data from an array of integers to simulate
' the memory of an old 8 bit computer. Bites are addressed
' 0 through to MaxBites.
' The routines use an integer array and require an integer
' of 0 through 255 to be stored or returned
' - no error checking included here.
' The misspelling of Byte to Bite was intentional so as
' not to confuse MMB.

' You should envisage the layout as follows

' Array(0) Bite 7 Bite 6 Bite 5 Bite 4 Bite 3 Bite 2 Bite 1 Bite 0
' Array(1) Bite 15 Bite 14 Bite 13 Bite 12 Bite 11 Bite 10 Bite 9 Bite 8
' Array(2) Bite 23 Bite 22 Bite 21 Bite 20 Bite 19 Bite 18 Bite 17 Bite 16
' .... and so on. Bite 0 being the LSB 8 bits of array(0), Bite 1 the next ...
'*************************************************************

' Global variables required by the two subroutines below
dim BiteCounter
dim MaxBites = 1024
dim BiteArray%(MaxBites \ 8 + 1)
dim Bite%

' Some test code.
' Example that fills the array with random data
for BiteCounter = 0 to MaxBites
local tstval
tstval = fix(rnd * 256)
Bite% = tstval
print "BiteCounter = ",hex$(BiteCounter),"Bite = ",hex$(Bite%)
PutBite BiteArray%(),BiteCounter,Bite%
next

' Example of pulling data from array
for BiteCounter = 0 to MaxBites
GetBite BiteArray%(),BiteCounter
print "BiteCounter = ",hex$(BiteCounter),"Bite = ",hex$(Bite%)
next
end

'*******************************************************************
' These are the two subroutines to do the work
' Put Bite into array
sub PutBite BiteArray%(),BiteCounter,Bite%
local PosMask,BiteArrayIndex,BiteShifted%,xx
PosMask = BiteCounter mod 8 ' which bite in the 64 bit integer?
BiteShifted% = Bite%
for xx = 0 to PosMask ' shift the bite into position in the integer
if xx <> 0 then ' correct for 0 to 0 giving 1 unwanted pass through
BiteShifted% = BiteShifted% << 8
endif
next xx
BiteArrayIndex = BiteCounter \ 8 ' calculate the index into the array
BiteArray%(BiteArrayIndex) = BiteArray%(BiteArrayIndex) or BiteShifted%
End sub

' Get Bite from array - see comments above
sub GetBite (BiteArray%(),BiteCounter)
local PosMask,BiteArrayIndex,BiteShifted%,xx
PosMask = BiteCounter mod 8
BiteShifted% = &HFF
for xx = 0 to PosMask
if xx <> 0 then
BiteShifted% = BiteShifted% << 8
endif
next xx
BiteArrayIndex = BiteCounter \ 8
Bite% = BiteArray%(BiteArrayIndex) AND BiteShifted%
End sub

... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it!

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