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A project log for Targeting SDCC to the 8080

Writing a code generator for the 8080 microprocessor for Small Device C Compiler (SDCC)

Ken YapKen Yap 10/09/2019 at 23:410 Comments

A binary compiled from C needs some additional code to work. These are the runtime library routines. Typically they consist of utility routines such as character and string functions that many C programmers expect as part of the environment. Even in embedded environments a rudimentary stdio library may be provided that at the bottom calls a couple of routines, getchar() and putchar(), which must be provided by the developer for a particular embedded platform say by talking to a serial interface.

The library may also contain assist functions to do operations not supported by the processor, such as multiplication and division. In our case right shift routines fall into this category. Incidentally have you ever wondered how the user is prevented from writing C functions and global variables that may override and interfere with these assist functions and variables? Originally in Unix this was done simply by prepending an underscore (_) to every external symbol. In other words, the C programmer can only create symbols in the object files starting with underscore. Assist functions and variables do not start with an underscore so cannot be overriden. If the user writes in assembler to be linked with the objects from C, then there is no barrier. In that case the user is assumed to know what she is doing.

Other things in the runtime library are routines written in assembler for speed, for example block copies and compares, accessed for example via bcopy() and bcmp().

But the one thing the runtime library must contain is the startup module. Traditionally this was named crt0.o in Unix (C runtime zero). This module accepts control from the OS, or from the boot vector and sets up the registers as necessary for the payload to run. The program counter is of course taken care of when the startup jumps to the payload. Other registers that must be set up include the stack pointer, any segment pointers, and interrupt vectors, this last for embedded environments. The C environment also stipulates that unintialised variables must contain binary zero. These variables are stored in the BSS area, typically above the code and initialised variables (read-only in recent C standards), but below the heap. The stack typically extends downward from the top of RAM. So one of the jobs the startup must do is zero the BSS.

Since embedded environments vary, there is no one size fits all crt0. Typically the compiler package provides a standard crt0, but the developer is expected to take the source and customise for the hardware configuration that will be used. In SDCC, like in traditional C compilers, it is possible to tell the linker to omit the provided crt0.o and then the developer ensures that the first file handed to the linker is her customised crt0.o.

The runtime library contains a mix of C and assembler. Assembler is used where C cannot or efficiency is crucial.

So TODO number 2 is to take the assembler routines that are written in Z80 code and produce equivalents in 8080 code for the 8080 runtime library. As an example, the Z80 startup will use the Z80 block assign opcodes to zero the BSS. The 8080 code should do this the longer way.

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