kvisc/ka/ABI

# The OS/K Team licenses this file to you under the MIT license.
# See the LICENSE file in the project root for more information.

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1. STACK

Stack grows downward. 'rbp' and 'rsp' are both used.
The lowest stack frame is marked by having 'rbp' = 0.
There is a 128-bytes red zone below 'rsp'.

A function's assembly code looks like this:
    label:
        enter N
        ...
        ...
        ...
        leave
        ret
'N' is the number of local variables used by the function.

The above code is equivalent to (and can be substitued by) the following:
    label:
        push rbp
        mov rbp, rsp
        sub rsp, N*8
        ...
        ...
        ...
        mov rsp, rbp
        pop rbp
        ret

Between the 'enter' and the 'leave', the stack looks like this:
        .               .
        .               .
        .   (caller's)  .
        |---------------|
        |   saved rip   |   rbp+8           rsp+(N*8)+16
        |   saved rbp   |   rbp             rsp+(N*8)+8
        |       .       |   rbp-8           rsp+(N*8)   \
        |       .       |   rbp-16          rsp+(N-1)*8 |
        |       .       |   ...                         | <- local
        |       .       |   rbp-(N-1)*8     rsp+8       | variables
        |       .       |   rbp-(N*8)       rsp         |
        |---------------|                               /
        .   (red zone)  .
        .               .
        .               .


'enter' and 'leave' can be omitted if ALL the following hold:
the function
    - uses no local variables (on the stack)
    - never uses any function that changes 'rbp' nor 'rsp',
      aside from 'call' and 'ret'

You can never omit 'enter' without omitting 'leave', and vice-versa.

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2. CALLING CONVENTION

No matter whether 'enter' and 'leave' were ommited or not, the following must
hold true whenever a function is entered or exited from:
    - 'rbp' must be 8-bytes aligned
    - 'rsp' must be 8-bytes aligned
    - the DF flag must be cleared

Aside from the DF flag, a function cannot assume anything about the state
of the flags in the FLG register.

Passing parameters is done using the following registers, in that order:
    ax0, ax1, ax2, ax3, ax4, ax5

Further parameters are passed on the stack in reverse order. The caller then 
cleans the stack.

Return values are passed in 'rax'. If the return value does not fit
and require more registers, use the following registers, in that order:
    rax, rdx

The following registers are volatile; the caller cannot assume
that they will be left unmodified by the callee:
    rax, rbx, rcx, rdx, rsi, rdi, ax0, ax1, ax2, ax3, ax4, ax5

The following registers are nonvolatile; the callee must preserve them:
    r12, r13, r14, r15, r16, r17, r18, r19, r20

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3. VARIADIC FUNCTIONS

To call a variadic function, do this:
        sub rsp, nargs * 8
        mov [rsp], arg0
        ...
        ...
        ...
        mov [rsp+(N*8)], argN
        call variadic_func
        add rsp, nargs * 8

To the variadic function, argN can be accessed the following way:
        mov reg, [rbp+N*8+16]

For instance:
        mov rax, [rbp+16]       ; arg0
        mov rdx, [rbp+24]       ; arg1

It is recommended to use the reg+reg*imm16+imm16 memory format:
        mov rax, [rbp+rcx*8+16]         ; accesses arg#rcx

The 'va_list' type can be regarded as a pointer to the
variadic function's rbp+16

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4. SPECIAL REGISTERS

(TO BE COMPLETED)

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