The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

The more complex bank function is function 0. This request is used to actually exchange
the current bank with the specified bank. A very important point to remember here is that
since a memory transfer will take place in the address range X'8000' to X'FFFF', the
transfer cannot proceed correctly if the stack pointer (SP) contains a value that places
the stack in that range. In fact, @BANK will inhibit function 0 and return an SVC
parameter error if the stack pointer violates the condition.
A bank can be used purely as a data storage buffer. Most likely, the application's
routines for invoking and indexing the bank switching will reside in the user range
X'3000' through X'7FFF' (or possibly in the I/O driver range). As an example
illustration, the following code will invoke a previously tested and reserved bank (via
functions 2 and 3), access the buffer, and then restore the previous bank:
LD C,1 ;Specify bank-1
LD B,0 ;Bring up bank
LD A,@BANK ;Identify the SVC
RST 40
JR NZ,ERROR ;Whatever error trap
PUSH BC ;Save old bank data
.
your code to access the buffer region
.
POP BC ;Recover old bank data
LD A,@BANK ;Identify the SVC
RST 40
JR NZ,ERROR ;Whatever error trap
Note that the @BANK function 0 conveniently returns a zero in register B to effect a
function 0 later, as well as provides the old bank number in register C. This means that
you only have to save register pair BC, pop it when you want to restore the previous
bank, and then issue the @BANK SVC.
Say you have a need to transfer to another bank from a routine that is executing in high
memory. Can this be done? Notice that the only limitation discussed was that the stack
must not be in high memory. The @BANK SVC function 0 does provide a technique for
automatically transferring to an address in the new bank. This technique is termed the
transfer function. It relies on the assumption that since you are managing the 32K bank,
your application should know exactly where it needs to transfer (i.e. where the
application originally placed the code to execute). The code to perform a bank transfer
is similar to the above. Register pair HL must be loaded with the transfer address,
Register C, which contains the bank number to invoke, must have its high order (bit-7)
set to indicate the TRANSFER activity. After the specified bank is enabled, control is
passed to the transfer address that was in HL. Upon entry to your routine in the new bank
(we will refer to it as "PROGRAM-B"), register HL will contain the old RETurn address so
that PROGRAM-B will know where to return to when it transfers. Register C will also
contain the old bank number with bit-7 set and register B will contain a zero. This
register setup will provide for an easy return to the routine in the old bank that
invoked the bank transfer. An illustration of the transfer code is as follows:
LD C,1 ;Specify bank-1
LD B,0 ;Bring up bank 0
LD HL,(TRAADR) ;Set the transfer address
SET 7,C ; & denote a transfer
LD A,@BANK ;Identify the SVC
RST 40
RETADR JR NZ,ERROR
Control will be returned to "RETADR" under either of two conditions. If there was an
error in executing the bank transfer (for instance an invalid bank number or the stack
pointer being in high memory), the returned condition will be NZ. If the transfer took
place and PROGRAM-B transferred back, the returned condition will always have the Z-flag
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