Sophie: sdcc-3.0.0-0.fc14 x86

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<LI><A NAME="tex2html2317"
  HREF="node106.html#SECTION00513100000000000000">4.1.3.1 Hardware</A>
<LI><A NAME="tex2html2318"
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<H2><A NAME="SECTION00513000000000000000"></A><A NAME="sub:Bankswitching"></A>
<BR>
4.1.3 Bankswitching
</H2>

<P>
Bankswitching<A NAME="2893"></A> (a.k.a. code banking<A NAME="2894"></A>)
is a technique to increase the code space above the 64k limit of the
8051.

<P>

<H3><A NAME="SECTION00513100000000000000">
4.1.3.1 Hardware</A>
</H3>

<P>
<TABLE CELLPADDING=3 BORDER="1">
<TR><TD ALIGN="CENTER" COLSPAN=1>8000-FFFF</TD>
<TD ALIGN="CENTER">bank1</TD>
<TD ALIGN="CENTER">bank2</TD>
<TD ALIGN="CENTER" COLSPAN=1>bank3</TD>
</TR>
<TR><TD ALIGN="CENTER" COLSPAN=1>0000-7FFF</TD>
<TD ALIGN="CENTER">common</TD>
<TD ALIGN="CENTER" COLSPAN=2>&nbsp;</TD>
</TR>
<TR><TD ALIGN="CENTER" COLSPAN=4>SiLabs C8051F120 example</TD>
</TR>
</TABLE>
<BR>
<BR>
Usually the hardware uses some sfr (an output port or an internal
sfr) to select a bank and put it in the banked area of the memory
map. The selected bank usually becomes active immediately upon assignment
to this sfr and when running inside a bank it will switch out this
code it is currently running. Therefor you cannot jump or call directly
from one bank to another and need to use a so-called trampoline in
the common area. For SDCC an example trampoline is in crtbank.asm
and you may need to change it to your 8051 derivative or schematic.
The presented code is written for the C8051F120.
<BR>
<BR>
When calling a banked function SDCC will put the LSB of the functions
address in register R0, the MSB in R1 and the bank in R2 and then
call this trampoline <I>__sdcc_banked_call</I>. The current selected
bank is saved on the stack, the new bank is selected and an indirect
jump is made. When the banked function returns it jumps to <I>__sdcc_banked_ret</I>
which restores the previous bank and returns to the caller.

<P>

<H3><A NAME="SECTION00513200000000000000">
4.1.3.2 Software</A>
</H3>

<P>
When writing banked software using SDCC you need to use some special
keywords and options. You also need to take over a bit of work from
the linker.
<BR>
<BR>
To create a function that can be called from another bank it requires
the keyword <I>banked</I><A NAME="2921"></A>. The caller must see this
in the prototype of the callee and the callee needs it for a proper
return. Called functions within the same bank as the caller do not
need the <I>banked</I> keyword nor do functions in the common area.
Beware: SDCC does not know or check if functions are in the same bank.
This is your responsibility!
<BR>
<BR>
Normally all functions you write end up in the segment CSEG. If
you want a function explicitly to reside in the common area put it
in segment HOME. This applies for instance to interrupt service routines
as they should not be banked.

<P>
Functions that need to be in a switched bank must be put in a named
segment. The name can be mostly anything up to eight characters (e.g.
BANK1). To do this you either use --codeseg BANK1 (See <A HREF="node54.html#lyx:-codeseg">3.2.9</A>)
on the command line when compiling or #pragma codeseg BANK1 (See
<A HREF="node100.html#sec:Pragmas">3.19</A>) at the top of the C source file. The segment name
always applies to the whole source file and generated object so functions
for different banks need to be defined in different source files.
<BR>
<BR>
When linking your objects you need to tell the linker where to
put your segments. To do this you use the following command line option
to SDCC: -Wl-b BANK1=0x18000 (See <A HREF="node48.html#lyx:-Wl_option">3.2.3</A>). This sets
the virtual start address of this segment. It sets the banknumber
to 0x01 and maps the bank to 0x8000 and up. The linker will not check
for overflows, again this is your responsibility.

<P>

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2011-03-20
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