the arm instruction set
the arm has six operation modes:
user (unprivileged mode under which most tasks run)
fiq (entered when a hight priority(fast) interrrupt is raised)
irq (entered whene a low priority(normal ) interrrupt is raised )
supervisor( entered on reset and when a software interrupt instruction is executed )
swi 指令
abort( used to handle memory access violations) 用于处理 memory access violations
undef( used to handle undefined instructions)
system (privileged mode using the same registers as user mode )
37个寄存器
1 pc
1 cpsr
5 spsr
30 通用寄存器
特权模式才有
spsr
r13 = sp
r14 = lr
user mode return address calculated from user mode pc value
and stored in fiq mode lr
user mode cpsr copied to fiq mode spsr
CPSR有4个8位区域 标志域（F-flag）、状态域（S-status）、扩展域（X-extention）、控制域（C-control）
C 控制域屏蔽字节(psr[7:0])
X 扩展域屏蔽字节(psr[15:8])
S 状态域屏蔽字节(psr[23:16])
F 标志域屏蔽字节(psr[31:24])
如: msr CPSR_fc, r0
n negative
z zero
c carried out
v verflowed
all instruction are 32 bits in length 32长度的指令
all instructions must be word aligned 字对齐
pc 值存储 31：2
1:0 equal to zero
r14 is used as the subroutine link register(LR) and stored the return address
whene branch with link operations are performed,
mov r15, r14
mov pc, lr : lr--->pc
0x0 reset
0x04 undefined instruction
0x08 software interrupt
0x0c prefetch abort
0x10 data abort
0x14 reservered
0x18 irq
0x1c fiq
异常的过程
copy cpsr to spsr_<mode>
set appropriate cpsr
maps in appropriate banked registers
stores the "return address" in lr_<mode>
sets pc to vector address
restore cpsr from spsr_<mode>
restore pc from lr_<mode>
the instruction pipeline
the arm used a pipeline in order to increase the speed of the flow of instruction to the processor
pc fetch instruction fetched from memory
pc-4 decode decoding of registers used in instruction
pc-8 execute register read from register bank shift and alu operation
pc rahter than pointing to the instruction being executed (不是指向要执行的) but points to the instruction being fetched
而是要取的指令