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- The m68k2 back end is an EM code generator for the
- Motorola MC68000. It defines an integer to be 16 bits
- and a pointer to be 32 bits.
- The m68k2 back end generates code for the ACK 68000 assembler.
- (The mnemonics recognized by this assembler can be found in
- "as/mach3.c").
- Some parts of the back end are system dependent, i.e. they depend
- on the kind of target 68000 system.
- - The way to do a Unix system call may vary from system to system.
- For every system call you need to have an assembly routine that
- passes the arguments and the system call number to Unix.
- These routines should be put in the library file "lib/tail_mon".
- The distribution contains a tail_mon file tailored for
- UniSoft Unix (see directory "libsys").
- Beware that several Unix systems (e.g. UniSoft Unix) use 4-byte
- integers, whereas the m68k2 back end produces code for 2-byte
- integers. In this case all system calls having an "int" parameter
- should convert their parameters to "long"s.
- - Most systems require some sort of "test for enough stack space"
- at the beginning of every procedure, to get around the "back up"
- problem. E.g. UniSoft Unix requires a "tst.b N(sp)" instruction
- This instruction is generated by the routines "prolog()" and
- "save" in "cg/mach.c".
- - The output of the ACK 68000 linker is an a.out file that
- has a different format as an a.out file on your system. (As most
- 68000 systems have different a.out formats, there is no such thing
- as "the" 68000 a.out format). So a program is needed to convert the
- ACK a.out format to your a.out format (as defined in
- "/usr/include/a.out.h"). The program "cv/cv.c" does
- the job for UniSoft Unix. It probably need only be slightly
- modified for your system.
- If your target 68000 does not run Unix, but is e.g. a stand alone
- 68000, you will need a program to download the ACK a.out file.
- The program "dl/dl.c" produces Intel Hex format on standard output
- from an a.out file.
- - The EM runtime start-off ("libsys/head_em.s") may have to be modified.
- It should call the procedure __m_a_i_n with parameters (argc,argv,envp).
- Usually, Unix will put these on top of the stack before starting
- the program. Note, however, that for 4-byte systems Unix will provide
- a 4-byte argc, while __m_a_i_n expects a 2-byte argc; so the value
- must be shortened to 2 bytes.
- The m68k2 code generator translates most EM instructions in line.
- For some complex EM instructions it uses assembly routines (stored in the
- library "libem/libem_s.a").
- The generated code does not check for array bound errors, overflow in
- arithmetic operations or division by zero (the latter will cause a hardware
- trap).
- The code generator has the following register conventions:
- a7: stack pointer
- a6: local base pointer
- a0,a1,d0,d1,d2: scratch registers
- (d0 is also used for 2/4 bytes function results;
- d0 and d1 are used for 8 bytes function results)
- a2-a5,d3-d7: register variables.
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