Config Base = 0 : Dimensionin SRAM & ERAM Order

DennisH · Joined: 25 Mar 2005 Posts: 13 Location: Melbourne Australia

In a program under development there is a subtle difference in the way Sram and Eram is dimensioned.

I am using a 256 byte rotary buffer starting on an even boundary, eg, H200. If the buffer is indirectly
addressed using the X register (eg, using $ASM code) only the lower address byte changes which
simplifies code.

However, if Eram is dimensioned before Sram the buffer starts at H201. If the Sram is dimensioned before
the Eram the buffer starts at H200 - which is my desired result.

Apart from the address anomaly, both methods appear to run OK.

My question is what constraints are there on the order of dimensioning re eram, sram, bits, bytes, words, etc.

The following code is a brief example:

$regfile = "m2560def.dat"

'$prog Lb Fb Fbh Fbx
'
'$prog &HFF , &HFF , &HD8 , &HFD

$crystal = 16000000
$hwstack = 128
$swstack = 128
$framesize = 128

' the order of dimensioning eram and ram matters

' is important because when addressing via X reg only lower byte of X changes
' when array address is on even (H00) boundary

'--- test sample 1: A at address H201 ---

'Config Base = 0

'Dim E As Eram Byte
'Dim W As Eram Word

'Dim A(256) As Byte
'Dim I As Byte

'--- test sample 2: A at address H200 ---

Config Base = 0

Dim A(256) As Byte
Dim I As Byte

Dim E As Eram Byte
Dim W As Eram Word

' code
A(0) = 100
Print A(0)

DennisH · Joined: 25 Mar 2005 Posts: 13 Location: Melbourne Australia

Update:

The problem seems to centre on the first dimensioning. Subsequent dimensions are correct.

This gives the required outcome:

Config Base = 0

Dim A(255) As Byte ' size changed to 255
Dim B(256) As Byte
Dim C(256) As Byte
Dim D(256) As Byte

Dim J As Word
Dim K As Byte

Dim E As Eram Byte
Dim W As Eram Word

-----------------------
' Memory allocations

A Byte(255) 0200 512
B Byte(256) 0300 768
C Byte(256) 0400 1024
D Byte(256) 0500 1280
J Word 0600 1536
K Byte 0602 1538
E EEPROM Byte 0000 0
W EEPROM Word 0001 1

albertsm · Joined: 09 Apr 2004 Posts: 5921 Location: Holland

you do not specify a version. so i will just focus on how it works in 2084.
some internal variables are dimensioned automatically depending on the used statements.
in order to get the first DIM statement starting on a boundary(that was required for xmega usb) the first user DIM statement will put memory on the first sram location. after that some internal variables could be located. for example config clock will create _day, _hour etc. variables.

you can always create one big array and use overlay to locate data inside this area.

DennisH · Joined: 25 Mar 2005 Posts: 13 Location: Melbourne Australia

Mark, thanks.

Sorry for omitting the version number. I get the same results with 2081 and 2084.

The big master array with overlays is a great idea.

I already use quite a few overlays in my code - I should have thought of that!

Dennis