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

8.8 MEMORY BANK SWITCHING
This section discusses the techniques of using the @BANK SuperVisor Call. The control of
an assembly-coded application operating in a memory banked environment requires a high
degree of skill in assembly language coding and should not be undertaken by the novice.
The professional is advised to carefully read the information contained in this section
which discusses how bank switching is supported within the operating system.
The DOS can support eight multiple RAM banks of 32K each in addition to a resident 32K
bank. This brings the total RAM configuration to 288K. The non-resident RAM banks are
designated as banks zero through seven. The 32K of bank zero (generally considered as
"high memory") and the resident 32K are considered the standard 64K of the DOS. Banks one
through seven may be used for buffers or data storage. Through sophisticated techniques,
they can even be used to store executable code. An entire bank is reserved for a
particular function. The system maintains a pointer (HIGH$) for bank 0 only. At any one
time, only one of the banks are resident. All are imaged at address X'8000' through
X'FFFF'. When a bank transfer is performed, the specified bank becomes addressable and
the previous bank is no longer available. Since memory refresh is performed on all banks,
nothing in the previously resident bank is altered during whatever time it is not
addressable (i.e. not resident).
The DOS provides support in accessing this additional RAM by means of the @BANK
SuperVisor Call (SVC-102). Let's take a look at how this RAM is handled. When the
operating system is booted, it examines what banks of RAM are installed in the machine.
The DOS maintains a byte bit-map with each bit representing one of the banks of RAM. This
byte is called "Bank Available RAM" (BAR), and its information is set when the DOS is
booted. BAR bit-0 corresponds to bank 0, BAR bit-1 corresponds to bank 1, and so on to
BAR bit-7 corresponding to bank 7. A machine may have only one bank, bank 0.
Another byte bit-map is used to indicate whether a bank is reserved or available for use.
This byte is designated the "Bank Used RAM" (BUR). Again, a bit assignment corresponds
one-for-one with the bank number. The management of any memory space within a particular
bank of RAM (excluding bank 0) is the sole responsibility of the application program
"reserving" a particular bank.
The DOS I/O device handler will always enable bank 0 upon execution of any character I/O
service request (@PUT, @GET, @CTL, as well as those other character I/O SVCs that use
@PUT/@GET/@CTL). The DOS also enables bank 0 at the initial entry to the task processor
and when a disk I/O communications function is requested. This requires that any device
driver or filter that is relocated to high memory (X'8000'-X'FFFF') must reside in bank
0. The same holds true for interrupt task routines and disk drivers/filters. The system
provides this restriction to make sure that any filter, driver, or task routine that
control passes to will be occupying enabled RAM memory. If a RAM bank other than 0 was
resident during these operations, it would be restored upon return from the
device/drive/task handler. The limitation will ensure that device I/O, task processing,
and disk I/O will never be impacted due to bank switching of RAM by an application.
Another restriction requires that the stack pointer (SP) is not pointing to an adddress
above X'7FFE' when a bank transfer is requested. This is because that stack range would
have placed the stack in the memory region that is being swapped thereby making the stack
contents erroneous. The @BANK SVC will inhibit the request and return an "SVC parameter
error" if this condition is violated. It is acceptable for an interrupt task, filter
module, or driver that is located in the bank switched address range to perform a bank
transfer to another bank provided the necessary linkage and stack area is being utilized.
This will be discussed later in more detail.
All bank transfer requests must be performed using the @BANK SVC. This SVC provides five
functions - four of which are interogatory in nature. One of the functions performs
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