Some Header Files

1
Some Header Files

• msoe/ports.h defines standard ports
• ports_fox11.h Fox11 ports (Dr. Durant)
• msoe/common.h defines
• byte unsigned char
• word unsigned short
• size_t long unsigned
• msoe/time.h delay routines
• msoe/display.h hex to ascii and other routines
  for Wookie/Briefcase display
• msoe/string.h simple string handling routines

2
Compiler Usage

• Located at c\usr\... ? you installed it first
  week (with assembler)
• bin executables
• man documentation
• lib libraries startup, std, msoe

3
More Usage

• For each C/C module
• gcc g mshort Wall fno-exceptions O2 S
  name.cpp
• as o name.o name.s
• Link
• ld
• -T (include our memory map file)
• -relax (optimize page0 addressing)
• crt1.o ? startup code
• list of .o files
• -L (library search directories)
• -l (other libraries)

4
Mixing AS and C/C

• Note, for C/C
• gcc g mshort Wall fno-exceptions O2 S
  name.cpp
• as o name.o name.s
• For each ASM module
• as o name.o name.s
• Simply link object files together
• We do need some glue code

5
C Calling Assembly Subroutines

• Linker will need find the routine at link time
• We need a placeholder so C can compile
• Do it with a prototype this tells C what data
  types to send the routine and what type to expect
  in return.
• Must declare assembly routines with extern C
• extern so that the compiler will defer resolution
  until link time
• C so that the compiler will assume C linkage
  convention
• Here C refers to the linkage convention of the
  C language, not the C language itself.
• C has a different and incompatible linkage
  convention due primarily to its overloading
  capabilities.
• We can wrap several declarations at once
• extern C
• declaration 1
• declaration
• Normally put in a header file
• Assembly must make entry point global
• Works for variables too

6
Example

• In C file
• extern "C" void flashLEDS(void)
• int main() // gcc 3 requires int return, but
  allows omission of return statement
• for() // a forever loop - common for
  embedded systems
• flashLEDS()
• In .S file
• .section .text
• .global flashLEDS
• flashLEDS
• ldaa 0xFF
• staa 0x1404
• NOP

7
Calling C From Assembly

• Again, C uses name mangling
• To allow for overloading, the entry point of a
  C is an encoding of its name and its signature.
• Different compilers may use different mangling
  algorithms, thus it is difficult to predict what
  a mangled name will be.
• Better to declare C routine that you want to
  call from assembly as extern C as well.

8
Example

• In Cpp File
• extern "C" void flashLEDS(void)
• extern "C" void flashLEDS2(void)
• void flashLEDS2(void)
• unsigned char LEDS (unsigned char )0x1404
• LEDS 0xFF
• LEDS 0x00
• int main() // gcc 3 requires int return, but
  allows omission of return statement
• for() // a forever loop - common for
  embedded systems
• flashLEDS()

9
What About Registers?

• Just like subroutines in assembly language, you
  want to avoid changing any side-effects, such as
  changing
• GCC doesnt care about A, B, D, X, or Y
  (documented) and CCR does not appear to be an
  issue either.

10
Passing Parameters

• Typically, parameters would be passed via
  registers.
• HC11 is very register poor.
• Uses register B or D for first argument
• Depending on type
• Remaining parameters are passed on the stack
• Return value is passed back in register B or D
• Depending on type

11
Getting Parameters in Assembly

• First parameter is
• in B if it is byte sized
• A is undefined and may not be 0
• in D if it is word sized
• Remember some parameters are passed by reference
  (char for example)
• The rest are on the stack
• All parameters consume two bytes or positions on
  the stack regardless of whether they are bytes or
  words
• The last position of the stack will have the
  2-byte return address needed by RTS
• Consider using the TSX instruction at the
  beginning of your assembly routine
• TXS - X now contains SP1, and points to MSB of
  return address
• LDAX 0,X refers to MSB of return address
• LDAX 1,X refers to LSB of return address
• LDAX 2,X refers to MSB of parameter 2 (not used
  if p2 is a byte)
• LDAX 3,X refers to LSB of parameter 2 (or p2 if
  it is a byte)
• LDAX 4,X refers to MSB of parameter 3 (not used
  if p3 is a byte)
• LDAX 5,X refers to LSB of parameter 3 (or p3 if
  it is a byte)

12
Returning Values

• Put return value in
• B if it is a byte
• D if it is a word
• What about float, double, long, etc?
• Compiler uses combination of D, X, and Y for
  parameters, and additional stack space
• Becomes difficult to track
• Compiler dependent
• Avoid if possible