linux 内核启动 lookup machine type 代码 问题

[东大坡居士]

__lookup_architecture_type:
	adr	r4, 2b
	ldmia	r4, {r2, r3, r5, r6, r7}	@ throw away r2, r3
	sub	r5, r4, r5			@ convert addresses
	add	r4, r6, r5			@ to our address space
	add	r7, r7, r5
1:	ldr	r5, [r4]			@ get machine type
	teq	r5, r1				@ matches loader number?
	beq	2f				@ found
	add	r4, r4, #SIZEOF_MACHINE_DESC	@ next machine_desc
	cmp	r4, r7
	blt	1b
	mov	r7, #0				@ unknown architecture
	mov	pc, lr
2:	ldmib	r4, {r5, r6, r7}		@ found, get results
	mov	pc, lr

一直搞不懂上面这段：
sub r5, r4, r5 @ convert addresses
add r4, r6, r5 @ to our address space
是怎么弄的，怎么转换的地址？ 谁能帮忙解释一下，谢谢