问题描述
我期待通过本教程：http://www.cl.cam.ac.uk/freshers/raspberrypi/tutorials/os/ok01.html
组件的第一行是：
ldr r0,=0x20200000
第二个是：
mov r1,#1
我觉得 LDR 是装载值从内存到寄存器中。但似乎在 = 表示0x20200000是一个值不是一个内存地址。两条线似乎装载的绝对值。
I thought ldr was for loading values from memory into registers. But it seems the = means the 0x20200000 is a value not a memory address. Both lines seem to be loading the absolute values.
推荐答案
这是一个把戏/快捷方式。比方说
It is a trick/shortcut. say for example
ldr r0,=main
会发生什么样的汇编程序将分配一个数据字，指令附近，但是指令路径之外
what would happen is the assembler would allocate a data word, near the instruction but outside the instruction path
ldr r0,main_addr
...
b somewhere
main_addr: .data main
现在展开招常数/立即数，尤其是那些不能放入一个举动立即指令：
Now expand that trick to constants/immediates, esp those that cannot fit into a move immediate instruction:
top:
add r1,r2,r3
ldr r0,=0x12345678
eor r1,r2,r3
eor r1,r2,r3
b top
然后组装拆卸
00000000 <top>:
0: e0821003 add r1, r2, r3
4: e59f0008 ldr r0, [pc, #8] ; 14 <top+0x14>
8: e0221003 eor r1, r2, r3
c: e0221003 eor r1, r2, r3
10: eafffffa b 0 <top>
14: 12345678 eorsne r5, r4, #125829120 ; 0x7800000
和你看到的汇编程序中加入了数据字给你，改变了LDR到pc相对一个给你。
and you see the assembler has added the data word for you and changed the ldr into a pc relative for you.
现在，如果您使用即时，它适合在一个MOV指令，然后根据汇编程序或许，肯定有，因为我使用，它变成了一个MOV我
now if you use an immediate that does fit in a mov instruction, then depending on the assembler perhaps, certainly with the gnu as I am using, it turned it into a mov for me
top:
add r1,r2,r3
ldr r0,=0x12345678
ldr r5,=1
mov r6,#1
eor r1,r2,r3
eor r1,r2,r3
b top
00000000 <top>:
0: e0821003 add r1, r2, r3
4: e59f0010 ldr r0, [pc, #16] ; 1c <top+0x1c>
8: e3a05001 mov r5, #1
c: e3a06001 mov r6, #1
10: e0221003 eor r1, r2, r3
14: e0221003 eor r1, r2, r3
18: eafffff8 b 0 <top>
1c: 12345678 eorsne r5, r4, #125829120 ; 0x7800000
所以它基本上是一个打字快捷方式，了解你给汇编的权力，找个地方粘常数，它通常做得很好，有时会抱怨，不知道我是否已经看到了挡不它安全。有时候，你需要在code进行.ltorg或.pool鼓励汇编找个地方。
So it is basically a typing shortcut, understand that you are giving the assembler the power to find a place to stick the constant, which it usually does a good job, sometimes complains, not sure if I have seen it fail to do it safely. Sometimes you need a .ltorg or .pool in the code to encourage the assembler to find a place.
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