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