The use of ax, dx, cx to pass the function arguments is not `fastcall`
convention in GCC, which only uses cx and dx. This calling convention
comes from the GCC -mregparm=3 option.
[1] https://en.wikipedia.org/wiki/X86_calling_conventions
Yeah, we just moved to C code and now assembly again :) First of all, we can see that `memcpy` and other routines which are defined here, start and end with the two macros: `GLOBAL` and `ENDPROC`. `GLOBAL` is described in [arch/x86/include/asm/linkage.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/linkage.h) which defines the `globl` directive and its label. `ENDPROC` is described in [include/linux/linkage.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/linkage.h) and marks the `name` symbol as a function name and ends with the size of the `name` symbol.
The implementation of `memcpy` is simple. At first, it pushes values from the `si` and `di` registers to the stack to preserve their values because they will change during the `memcpy`. `memcpy` and other functions in copy.S use `fastcall` calling conventions. So it gets its incoming parameters from the`ax`, `dx` and `cx` registers. Calling `memcpy` looks like this:
The implementation of `memcpy` is simple. At first, it pushes values from the `si` and `di` registers to the stack to preserve their values because they will change during the `memcpy`. As we can see in the `REALMODE_CFLAGS` in `arch/x86/Makefile`, the kernel build system uses the `-mregparm=3` option of GCC, so functions get the first three parameters from`ax`, `dx` and `cx` registers. Calling `memcpy` looks like this:
```c
memcpy(&boot_params.hdr, &hdr, sizeof hdr);
@ -304,7 +304,7 @@ GLOBAL(memset)
ENDPROC(memset)
```
As you can read above, it uses the `fastcall` calling conventions like the `memcpy` function, which means that the function gets its parameters from the `ax`, `dx` and `cx` registers.
As you can read above, it uses the same calling conventions as the `memcpy` function, which means that the function gets its parameters from the `ax`, `dx` and `cx` registers.
The implementation of `memset` is similar to that of memcpy. It saves the value of the `di` register on the stack and puts the value of`ax`, which stores the address of the `biosregs` structure, into `di` . Next is the `movzbl` instruction, which copies the value of `dl` to the lower 2 bytes of the `eax` register. The remaining 2 high bytes of `eax` will be filled with zeros.
Iru Cai
6 years ago