This repository contains a non-official port. Original notices are kept in README.upstream and COPYING*.

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AMO Binutils (port)

GNU binutils port for the AMO architecture (provides as, ld, objdump, objcopy, …).

Target triple: amo-linux (amo-unknown-linux)

Target & ABI

Word size: 32-bit

Endianness: Little-endian

Alignment: word-aligned preferred; unaligned loads are hardware-supported by the SoC (one CPU cycle; handled by a 2-beat merge in the memory subsystem).

Calling convention:

Register	Special	Width	Notes
R0		32-bit	Caller-saved/Result register
R1-R15		32-bit	Caller-saved registers
R16-R25		32-bit	Callee-saved registers
R26-R27		32-bit	Interrupt registers
R28		32-bit	Argument register
R29	FP	32-bit	Frame pointer
R30	SP	32-bit	Stack pointer
R31	LR	32-bit	Link register
PC		32-bit	Program Counter
CPSR		4-bit	Current Program Status Register
IDTR		32-bit	Interrupt Descriptor Table Register

PC, CPSR are non-allocatable.

Instruction Set Architecture

Please refer to ISA.pdf for the detailed ISA specification.

Relocations

Reloc	Meaning
R_AMO_LITERAL	21-bit PC-relative (literal pool)
R_AMO_PCREL	16-bit PC-relative
R_AMO_28	28-bit Absolute
R_AMO_32	32-bit Absolute

Assembler

.global start
.extern kernel_stack
.section .inittext
; entry for kernel
start:
	; stack
	mov sp, kernel_stack
	add sp, sp, $4096
	mov fp, sp

	sub sp, sp, $4
	str [sp], lr

	; jump to c-based kernel
	jal kernel_init

	ldr lr, [sp]
	add sp, sp, $0x4

	; jump to bios section
	jmp lr

.ltorg

amo-linux-as boot.s -o boot.o

You can assemble the snippet above with as and inspect the output with objdump as shown below.

amo-linux-objdump -D boot.o

boot/boot.o:     file format elf32-amo


Disassembly of section .inittext:

00000000 <start>:
   0:	ldr  sp, [$0x20]                          20 00 c0 5b 	
   4:	add  sp, sp, $0x1000 (4096)               00 10 de 03 	
   8:	mov  fp, sp                               00 00 be 57 	
   c:	sub  sp, sp, $0x4 (4)                     04 00 de 13 	
  10:	str  [sp], lr                             00 00 df 67 	
  14:	jal  $0x0                                 00 00 00 80 	
  18:	ldr  lr, [sp]                             00 00 df 5f 	
  1c:	add  sp, sp, $0x4 (4)                     04 00 de 03 	
  20:	jmp  lr                                   00 00 e0 7f 	

00000024 <.__litpol_chunk_0_>:
  24:	add  r0, r0, $0x0 (0)                     00 00 00 00 	

Linker

OUTPUT_FORMAT(elf32-amo)
OUTPUT_ARCH(amo)
ENTRY(start)

SECTIONS
{
	. = 0x2400;

	.text :
	{
		*(.inittext)
		*(.text)
	}
	.rodata :
	{
		*(.rodata)
	}
	.data :
	{
		*(.data)
	}
	.bss :
	{
		*(COMMON)
		*(.bss)
	}

	_kernel_end = .;
}

amo-linux-ld -T linker/setup.ld boot.o -o boot

Flexible linking is supported with ld.

Build

configure --target=amo-linux --prefix=/path/install
make all -j8
make install