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@ -2,7 +2,7 @@
This book is written to the Linux calling convention as stated early on. This book is written to the Linux calling convention as stated early on.
Unfortunately, this means that even if you own an Apple Silicon machine, Unfortunately, this means that even if you own an Apple Silicon machine,
which is AARCH64, you'd still need a Linux virtual machine. which is AARCH64, you'd still need a Linux virtual machine.
This didn't sit well with some on reddit and rightfully so. We undertook This didn't sit well with some on reddit and rightfully so. We undertook
to develop a way of writing assembly code once and having it work on to develop a way of writing assembly code once and having it work on
@ -62,6 +62,13 @@ file ends in .S*
## Differences between Apple and Linux ## Differences between Apple and Linux
### Getting the address of `errno`
`errno` is an externally defined `int32_t` used by many "system"
provided APIs to report error conditions back to calling programs. The
macro `ERRNO_ADDR` can be used to converge how Linux and Apple get the
address of the variable (which is left in `x0`).
### Variadic functions ### Variadic functions
*This is important! Understand this section in order to be able to use *This is important! Understand this section in order to be able to use
@ -142,7 +149,7 @@ stack pointer. This is faster as the addition makes no reference to RAM
Apple requires that `x29` be kept as a valid stack frame pointer. The Apple requires that `x29` be kept as a valid stack frame pointer. The
frame pointer should always start out as equal to the stack pointer. frame pointer should always start out as equal to the stack pointer.
However, within the function, the stack pointer is free to change. The However, within the function, the stack pointer is free to change. The
frame pointer must remain fixed so that debuggers always know how to frame pointer must remain fixed so that debuggers always know how to
find the initial stack *frame*. find the initial stack *frame*.
To be Apple compatible, in addition to backing up `x30` also back up To be Apple compatible, in addition to backing up `x30` also back up