126 lines
3.8 KiB
Plaintext
126 lines
3.8 KiB
Plaintext
# The OS/K Team licenses this file to you under the MIT license.
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# See the LICENSE file in the project root for more information.
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#------------------------------------------------------------------------------#
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1. STACK
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Stack grows downward. 'rbp' and 'rsp' are both used.
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The lowest stack frame is marked by having 'rbp' = 0.
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There is a 128-bytes red zone below 'rsp'.
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A function's assembly code looks like this:
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label:
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enter N*8
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...
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...
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...
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leave
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ret
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'N' is the number of local variables used by the function.
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'N' may be omitted if it is zero.
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The above code is equivalent to (and can be substitued by) the following:
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label:
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push rbp
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mov rbp, rsp
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sub rsp, N*8
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...
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...
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...
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mov rsp, rbp
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pop rbp
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ret
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Between the 'enter' and the 'leave', the stack looks like this:
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. .
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. .
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. (caller's) .
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|---------------|
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| saved rip | rbp+8 rsp+(N*8)+16
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| saved rbp | rbp rsp+(N*8)+8
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| . | rbp-8 rsp+(N*8) \
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| . | rbp-16 rsp+(N-1)*8 |
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| . | ... | <- local
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| . | rbp-(N-1)*8 rsp+8 | variables
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| . | rbp-(N*8) rsp |
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|---------------| /
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. (red zone) .
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. .
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. .
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'enter' and 'leave' can be omitted if ALL the following hold:
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the function
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- uses no local variables (on the stack)
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- never uses any function that changes 'rbp' nor 'rsp',
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aside from 'call' and 'ret'
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You can never omit 'enter' without omitting 'leave', and vice-versa.
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#------------------------------------------------------------------------------#
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2. CALLING CONVENTION
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No matter whether 'enter' and 'leave' were ommited or not, the following must
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hold true whenever a function is entered or exited from:
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- 'rbp' must be 8-bytes aligned
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- 'rsp' must be 8-bytes aligned
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- the DF flag must be cleared
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Aside from the DF flag, a function cannot assume anything about the state
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of the flags in the FLG register.
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Passing parameters is done using the following registers, in that order:
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ax0, ax1, ax2, ax3, ax4, ax5
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Further parameters are passed on the stack in reverse order. The caller then
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cleans the stack.
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Return values are passed in 'rax'. If the return value does not fit
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and require more registers, use the following registers, in that order:
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rax, rdx
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The following registers are volatile; the caller cannot assume
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that they will be left unmodified by the callee:
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rax, rbx, rcx, rdx, rsi, rdi, ax0, ax1, ax2, ax3, ax4, ax5
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The following registers are nonvolatile; the callee must preserve them:
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r12, r13, r14, r15, r16, r17, r18, r19, r20
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#------------------------------------------------------------------------------#
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3. VARIADIC FUNCTIONS
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To call a variadic function, do this:
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sub rsp, nargs * 8
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mov [rsp], arg0
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...
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...
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...
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mov [rsp+(N*8)], argN
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call variadic_func
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add rsp, nargs * 8
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To the variadic function, argN can be accessed the following way:
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mov reg, [rbp+N*8+16]
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For instance:
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mov rax, [rbp+16] ; arg0
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mov rdx, [rbp+24] ; arg1
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It is recommended to use the reg+reg*imm16+imm16 memory format:
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mov rax, [rbp+rcx*8+16] ; accesses arg#rcx
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The 'va_list' type can be regarded as a pointer to the
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variadic function's rbp+16
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#------------------------------------------------------------------------------#
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4. SPECIAL REGISTERS
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(TO BE COMPLETED)
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#------------------------------------------------------------------------------#
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