ABI Interface - Revision history

Jcmvbkbc: fix ordinary call return value description

2018-11-10T01:17:58Z

fix ordinary call return value description

← Older revision		Revision as of 01:17, 10 November 2018
Line 1:		Line 1:
	== Argument Passing ==		== Argument Passing ==

	Arguments are passed in both registers and memory. The first six incoming arguments are stored in registers a2 through a7, and additional arguments are stored on the stack starting at the current stack pointer a1. Because Xtensa uses register windows that rotate during a function call, outgoing arguments that will become the incoming arguments must be stored to different register numbers. Depending on the call instruction and, thus, the rotation of the register window, the arguments are passed starting starting with register a(2+N), where N is the size of the window rotation. Therefore, the first argument in case of a <code>call4</code> instruction is placed into a6, and for a <code>call8</code> instruction into a10. Large arguments (8-bytes) are always passed in an even/odd register pair even if that means to omit a register for alignment. The return values are stored in a2 through a7.		Arguments are passed in both registers and memory. The first six incoming arguments are stored in registers a2 through a7, and additional arguments are stored on the stack starting at the current stack pointer a1. Because Xtensa uses register windows that rotate during a function call, outgoing arguments that will become the incoming arguments must be stored to different register numbers. Depending on the call instruction and, thus, the rotation of the register window, the arguments are passed starting starting with register a(2+N), where N is the size of the window rotation. Therefore, the first argument in case of a <code>call4</code> instruction is placed into a6, and for a <code>call8</code> instruction into a10. Large arguments (8-bytes) are always passed in an even/odd register pair even if that means to omit a register for alignment. The return values are stored in a2 through a5 (so a function with return value occupying more than 2 registers may not be called with call12).

	return addr stack ptr arg0, arg1, arg2, arg3, arg4, arg5		return addr stack ptr arg0, arg1, arg2, arg3, arg4, arg5

Jcmvbkbc: Fix syscall return value description.

2018-11-09T23:46:57Z

Fix syscall return value description.

← Older revision		Revision as of 23:46, 9 November 2018
Line 20:		Line 20:
	a2 a6, a3, a4, a5, a8, a9		a2 a6, a3, a4, a5, a8, a9

	Upon return, a2 contains the return value~~. A value of -1 indicates an error and the~~ error code ~~(<code>errno</code>) is stored in a3~~. All other registers are preserved.		Upon return, a2 contains the return value or error code. All other registers are preserved.

Chris at 18:34, 15 March 2013

2013-03-15T18:34:35Z

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Chris: New page: == Argument Passing == Arguments are passed in both registers and memory. The first six incoming arguments are stored in registers a2 through a7, and additional arguments are stored on th...

2007-04-24T07:00:16Z

New page: == Argument Passing == Arguments are passed in both registers and memory. The first six incoming arguments are stored in registers a2 through a7, and additional arguments are stored on th...

New page

== Argument Passing ==

Arguments are passed in both registers and memory. The first six incoming arguments are stored in registers a2 through a7, and additional arguments are stored on the stack starting at the current stack pointer a1. Because Xtensa uses register windows that rotate during a function call, outgoing arguments that will become the incoming arguments must be stored to different register numbers. Depending on the call instruction and, thus, the rotation of the register window, the arguments are passed starting starting with register a(2+N), where N is the size of the window rotation. Therefore, the first argument in case of a <code>call4</code> instruction is placed into a6, and for a <code>call8</code> instruction into a10. Large arguments (8-bytes) are always passed in an even/odd register pair even if that means to omit a register for alignment. The return values are stored in a2 through a7.

return addr stack ptr arg0, arg1, arg2, arg3, arg4, arg5
----------- --------- ----------------------------------
a0 a1 a2, a3, a4, a5, a6, a7

call4 a4 a5 a6, a7, a8, a9, a10, a11
call8 a8 a9 a10, a11, a12, a13, a14, a15
call12 a12 a13 a14, a15 --- --- --- ---

== Syscall ABI ==

Linux takes system-call arguments in registers. The ABI and Xtensa software conventions require the system-call number in a2. For improved efficiency, we try not to shift all parameters
one register up to maintain the original order. Register a2 is, therefore, moved to a6, a6 to a8, and a7 to a9, if the system call requires these arguments.

syscall number arg0, arg1, arg2, arg3, arg4, arg5
-------------- ----------------------------------
a2 a6, a3, a4, a5, a8, a9

Upon return, a2 contains the return value. A value of -1 indicates an error and the error code (<code>errno</code>) is stored in a3. All other registers are preserved.