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Added Corrected Documentation

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Thomas Fehmel
2016-10-24 10:33:47 +02:00
parent 657a54ba18
commit a126f7e01f
9 changed files with 28 additions and 4 deletions
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19
documentation/architecture.tex
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@@ -102,8 +102,23 @@ constant pc_num : reg_number := to_reg_number(15);
Registers \inlineasm{r0} to \inlineasm{r11} can be used without special consideration as general purpose registers.
Registers \inlineasm{r12} (stack pointer) and \inlineasm{r13} (link register) can also be used as general purpose register but are additionally altered with special instructions.
\subsection{Stack Pointer}
The stackpointer (\inlineasm{r12}) is used when jumping into or out of interrupt handlers (i.e. external interrupts or \inlineasm{trap} instructions and \inlineasm{reti}).
It must be set to the correct address of the stack, before any interrupt occurs.
The stack pointer (\inlineasm{r12}) is used when jumping into or out of interrupt handlers (i.e. external interrupts or \inlineasm{trap} instructions and \inlineasm{reti}).
It must be set to the correct address of the stack, before any interrupt is allowed to occur.
In the default configuration, stack grows downwards.
The stack pointer should be word-aligned at all times.
The interpretation of the stack pointer is always 'The current stack pointer points to the topmost element on the stack'.
When interfacing the stack pointer manually, always adhere to the following rules:
\begin{itemize}
\nolistskip
\item When writing to the stack (push), always decrement the stack before writing your element.
\item When reading from the stack (pop), read your element, then decrement the stack pointer.
\end{itemize}
Only when adhering to these rules the integrity of the stack can be ensured.
The initial stack pointer position does not need to be a memory address,
but the word address below it needs to be valid.
\subsection{Link Register}
The link register (\inlineasm{r13}) is set by the \inlineasm{call} instruction.
This allows for flexible function returns using the pseudo instruction \inlineasm{ret} (a branch to the link register).