计算机系统概论ICS Chapter 6 and after

Data Structure

用汇编实现...

Subroutine,JSR,JSRR,RET

汇编中的"函数" 调用函数用的是J

JSR LABEL 将原来PC的值放入R7，跳到LABEL对应的命令（PC+offset)
RET 从子过程中返回，将PC设为R7 相当于JMP R7
JSRR Rn 将原来PC的值放入R7,跳到Rn存储的地址(baseR)
JMP Rn只跳转PC=Rn,但不会保存原来PC

Call-return:

;main code
JSR SUBPROC
HALT

SUBPROC ;subprocess code
;code...
;用BR跳到ENDSUB可以在中途返回
;
ENDSUB RET

Caller Save/Callee Save

子过程的计算过程中需要寄存器，防止对主过程干扰。在子过程中先把原来寄存器的值存在内存里，计算完成后再恢复寄存器的值 局部变量要Save,返回值不用Save Figure 8.1

SUBPROC ST R1,SaveR1
        ;....code
        LD R1,SaveR1
        RET

SaveR1 .BLKW 1

Stack

Figure 8.11 P359 Overflow/Underflow

R6 ;stack pointer  (push -1, pop +1)
R0 ;value
R5 ;errorinfo

ADD R6,R6,#-1;
STR R0,R6,#0 ;(push)

Queue

FRONT: point to the location just before the first element of the queue.
REAR: point to the location containing the most recent element that was added to the queue
Empty: FRONT==REAR
Overflow FRONT=REAR+1

Recursion

如果连续调用了多个/次subroutine,只使用JSR和RET是回不去的,因为PC寄存器只有一个，只能回到上一个调用JSR的地方然后无限循环 利用栈进行函数调用 R6是栈指针

把当前过程用到的所有局部变量，指令存储的地址(JSR中被放到R7)，函数的返回值都放到栈上

比如求阶乘 ans=f(n-1)*n 就需要放ans,n,R7

I/O

LC-3 memory

x3000-xFE00

memory-mapped I/O: - Address control logic 控制访问的是内存，还是设备寄存器 - Addresses xFE00 to xFFFF are reserved for input/output device registers - KBDR xFE02 KBSR xFE00

Supervisor Stack Pointer (SSP) and User Stack Pointer (USP)

Register 6 is generally used as the stack pointer (SP) for the active stack.
Two registers, Saved_SSP and Saved_USP , are provided to save the SP not in use.

PSR

Privilege: the right to do something, such as execute a particular instruction or access a particular memory location.

Priority: the urgency of a program to execute.

两个概念没有关系。程序可以有高Priority,但低privilege(比如访问不了系统空间)

PSR[15]=0 means superisor privilege 注意0是supervisor,1才是用户

Basic Input-Output

异步asynchronous: 输入不会按CPU的时钟周期来

interrupt-driven:
polling:

KBSR xFE00KBDR xFE02 DSR xFE04 DDR xFE06

Each time the typist types a character, KBSR[15] = 1. Each time the computer reads a character, KBSR[15] =0(硬件自动实现)
显示器空闲时,DSR[15]=1。显示器正在处理输出时,DSR[15]=0

Manual I/O code

.ORIG x2000 ;在系统区里，否则没有supervisor权限
            ;如果放在x3000,访问KBSR会出现Access Violation
START LDI R3,KBSR; .FILL命令,mem[KBSR]=xFE00,R3=mem[mem[KBSR]]
      BRzp START ;test R3 input,如果第一位是0,就循环
      LDI R0,KBDR
      BRnzp NEXT_TASK
KBSR .FILL xFE00
KBDR .FILL xFE02


START STI R3,DSR
      BRzp START
      STI R0,DDR

KBSR[14]是interrupt enable bit，设为1时输入会导致中断，跳到x0180

IO实现的一些细节

回显：直接读完KBDR后显示是没有任何变化的，读取完后再将读取的字符输出，这样的显示效果才符合我们平时用命令行的操作

TRAP

程序请求操作系统接管，执行一些操作(service routine)。这些操作是写好存在内存里的

Trap vector: 8bit \(2^8=256\)

Trap vector table: starting addresses of these 256 service routines.(0000-00FF)

TRAP instruction

Execute phase:

PSR and PC are both pushed onto the system stack (a linkage back to the user program)
- If the the program is executing in User mode(PSR[15]=1), the USP must be saved and the SSP load
  1. PSR[15]=0
  2. PC=mem[ZEXT(trapvector)]

RTI instruction

opcode=1000, no operands

pop PC and PSR
set PSR[15] 0 or 1
If [PSR[15]]=1, load USP and save SS

SP是当前的栈指针,一般用R6

Saved_SSP和Saved_USP是两个寄存器 (见Figure C.8

Trap访问内存3次(两个入栈，一个读取地址)

Trap vector及其功能

mem[x0020]里面存的是x0300,对应输入操作的起始地址

HALT把MCR的第一位设为0

Interrupt

Trap是软件请求操作系统接管（软中断)

Interrupt是硬件(如IO)导致的中断 (硬中断)

Interrupt Vector Table: x0100-x01FF

跳转过程和Trap类似，看Figure C.7

Exception

对应Interrupt vector x0100,x0101,x0102

The privilege mode violation: occurs if the processor attempts to execute the RTI instruction while running in User mode.
The illegal opcode exception occurs if the processor attempts to execute an instruction having the unused opcode (bits [15:12] = 1101).
The ACV exception occurs if the processor attempts to access privileged memory (i.e., a memory location in system space or a device register having an address from xFE00 to xFFFF while running in User mode

TRAP 回到PC

Interrupt回到PC-1 (当前指令)，Exception回到PC-1 （当前引起异常的指令)