Transcript CS61C - Lecture 13

Machine Structures
Lecture 17 –
Introduction to CPU Design
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Five Components of a Computer
Computer
Processor Memory
(passive)
Control
(where
programs,
data live
Datapath when
running)
Devices
Input
Output
Keyboard,
Mouse
Disk
(where
programs,
data live
when not
running)
Display,
Printer
The CPU
• Processor (CPU): 计算机的核心，完成所
有的工作 (操作数据及决策)
• Datapath（数据通道）: processor的一部
分，功能是执行运算 (肌肉部分brawn)
• Control: processor的一部分，指挥（控制
）datapath做什么 (大脑部分brain)
Stages of the Datapath : Overview
• 问题: 将 “执行整个指令”的块做为一个整
体
• 太大(该块要执行从取指令开始的所有操作)
• 效率不高
• 解决方案: 将 “执行整个指令” 的操作分解
为多个阶段(stage)，然后将所有阶段连接
在一起产生整个datapath
• 每一阶段更小，从而更容易设计
• 方便优化其中一个阶段，而不必涉及其他阶
段
Stages of the Datapath (1/5)
• MIPS有多种指令: 共同的步骤是些什么?
• Stage 1: 取指
• 无论何种指令, 首先必须把32-位指令字从内
存中取出。(可能涉及缓存结构)
• 在这一步，我们还需要增加PC
(即PC = PC + 4, 以指向下一条指令，由于是
按字节寻址，故+4)
Stages of the Datapath (2/5)
• Stage 2: 指令译码Instruction Decode
• 在取到指令后, 下一步从各域(fields)中得到
数据(对必要的指令数据进行解码)
• 首先，读出Opcode，以决定指令类型及字
段长度
• 接下来，从相关部分读出数据
 for add, read two registers
 for addi, read one register
 for jal, no reads necessary
Stages of the Datapath (3/5)
• Stage 3: ALU (Arithmetic-Logic Unit)
• 大多数指令的实际工作在此部完成: 算术指令
(+, -, *, /), shifting, logic (&, |),
comparisons (slt)
• what about loads and stores?
 lw
$t0, 40($t1)
 要访问的内存地址 = $t1的值 + 40
 so we do this addition in this stage
Stages of the Datapath (4/5)
• Stage 4: 内存访问Memory Access
• 事实上只有load和store指令在此stage会做
事; 其它指令在此阶段空闲idle或者直接跳过
本阶段
• 由于load和store需要此步，因此需要一个专
门的阶段 stage来处理他们
• 由于cache系统的作用，该阶段有望加速
• 如果没有caches，本阶段stage会很慢
Stages of the Datapath (5/5)
• Stage 5: 写寄存器Register Write
• 大多数指令会将计算结果写到寄存器
• 例如: arithmetic, logical, shifts, loads, slt
• what about stores, branches, jumps?
 don’t write anything into a register at the end
 these remain idle during this fifth stage or
skip it all together
+4
1. Instruction
Fetch
ALU
Data
memory
rd
rs
rt
registers
PC
instruction
memory
Generic Steps of Datapath
imm
2. Decode/
Register
Read
3. Execute 4. Memory
5. Reg.
Write
Datapath Walkthroughs (1/3)
• add
$r3,$r1,$r2 # r3 = r1+r2
• Stage 1: 取指, 增加PC
• Stage 2: 解码，知道是add指令, 读寄存器
$r1 和 $r2
• Stage 3: 将上一步获得的两个值相加
• Stage 4: idle (不必读写内存)
• Stage 5: 将第三步Stage 3的结果写入寄存
器 $r3
+4
imm
reg[1]
reg[1]+reg[2]
reg[2]
ALU
Data
memory
2
registers
3
1
add r3, r1, r2
PC
instruction
memory
Example: add Instruction
Datapath Walkthroughs (2/3)
• slti
$r3,$r1,17
• Stage 1: 取指, 增加PC
• Stage 2: 解码，知道是slti, 然后读寄存器
$r1
• Stage 3: 比较上一步获得的值和17
• Stage 4: idle
• Stage 5: 将第三步的结果写入寄存器$r3
imm
17
reg[1]
reg[1]<17?
ALU
Data
memory
registers
+4
3
x
1
slti r3, r1, 17
PC
instruction
memory
Example: slti Instruction
Datapath Walkthroughs (3/3)
• sw
$r3, 17($r1)
• Stage 1: 取指, 增加PC
• Stage 2: 解码，知道是sw, 然后读寄存器$r1
和$r3
• Stage 3: 将17与寄存器 $r1的值相加 (上一步
获得)
• Stage 4: 将寄存器$r3的值(第2步取得)写到
第3步计算得到的内存地址
• Stage 5: idle (不必写入寄存器)
3
registers
+4
x
1
imm
17
reg[1]+17
reg[3]
ALU
Data
MEM[r1+17]<=r3 memory
SW r3, 17(r1)
instruction
memory
PC
Example: sw Instruction
reg[1]
Why Five Stages? (1/2)
• 是否能有不同的步骤?
• 是, 其他结构是这样的
• 为什么MIPS有５步，如果指令至少在某
一步空闲(idle)?
• 5步可以将所有的操作统一.
• There is one instruction that uses all five
stages: the load
Why Five Stages? (2/2)
• lw
$r3, 17($r1)
• Stage 1: 取指, 增加PC
• Stage 2: 解码，知道是lw, 读寄存器$r1
• Stage 3: 将17与寄存器 $r1的值相加 (上一
步得到)
• Stage 4: 从上一步计算得到的内在地址中读
值
• Stage 5: 将上一步得到的值写入寄存器$r3
3
registers
+4
LW r3, 17(r1)
instruction
memory
PC
x
1
imm
17
reg[1]+17
ALU
MEM[r1+17]
Data
memory
Example: lw Instruction
reg[1]
Datapath Summary
• 为了执行指令，需要有基于数据变换的数
据通道(Datapath)
+4
imm
opcode, funct
Controller
ALU
Data
memory
rd
rs
rt
registers
PC
instruction
memory
• 控制器controller 产生正确的变换
What Hardware Is Needed? (1/2)
• PC寄存器：用于踊跃记录下一个指令的内
存地址
• 通用寄存器
• 用于第二步 (Read) 和第五步(Write)
• MIPS has 32 of these
• 内存
• 用于第一步 (Fetch) 和第 4 步(R/W)
• Cache系统使得这两步和其他步骤同样快（
平均而言）
What Hardware Is Needed? (2/2)
• ALU
• 用于第三步
• 用于执行所有必要的函数功能: arithmetic, logicals,
etc.
• 后面会进行详细设计
• 其他寄存器
• 为了实现每个时钟周期执行一步, 在各步(stage)之间
插入寄存器以保存阶段变换过程中的中间数据和控
制信号.
• 注: 寄存器是通用名词，意即保存位的实体. 不是所
有寄存器都在“寄存器文件”中.
CPU clocking (1/2)
对每个指令, 如何控制数据通道中信息的流动?
• 单周期CPU: 指令的所有阶段在一个长的时钟周
期中完成.
• The clock cycle is made sufficient long to allow
each instruction to complete all stages without
interruption and within one cycle.
1. Instruction
Fetch
2. Decode/
Register
Read
3. Execute 4. Memory
5. Reg.
Write
CPU clocking (2/2)
对每个指令, 如何控制数据通道中信息的流动?
• 多时钟周期CPU: 每个时钟周期，执行一个
stage指令.
• 时钟和最慢的stage一样长.
1. Instruction
Fetch
2. Decode/
3. Execute 4. Memory
Register
Read
5. Reg.
Write
和单时钟执行相比，有几个好处: 某个指令未用的阶
段stages可以跳过，指令可以进入流水线pipelined
(重叠).
Verilog big idea: Time in code
• One difference from a prog. lang. is that
time is part of the language
• part of what trying to describe is when
things occur, or how long things will take
• In both structural and behavioral
Verilog, determine time with #n : event
will take place in n time units
• structural: not #2(notX, X) says notX does
not change until time advances 2 ns
• assign #2 Z = A ^ B; says Z does not
change until time advances 2 ns
• Default unit is nanoseconds; can change
2-input Mux with delay
module mux2 (in0, in1, select, out);
input in0,in1,select;
output out;
wire s0,w0,w1;
not
#1 (s0, select); // 1ns gate delays
and
#1 (w0, s0, in0),
(w1, select, in1);
or
#1 (out, w0, w1);
endmodule // mux2
Testing in Verilog
• Code examples so far define hardware
modules.
• Need separate code to test the module
(just like C/Java)
• Since hardware is hard to build, major
emphasis on testing in HDL
• Testing modules called “test benches”
in Verilog;
• like a bench in a lab dedicated to testing
• Could design special hardware blocks
to test other blocks - awkward! Use
behavioral Verilog
Example: Behavioral Test Block (signal generator)
Testing Verilog
• Create a test module for mux2:
module testmux;
reg a, b, s;
reg expected; wire f;
mux2 myMux(.select(s), .in0(a),
.in1(b), .out(f));
/* add testing code */
endmodule
• Outline: declare variable to use for connection
from testbench, instantiate module, specify
stimulus, (compare output to expected), print
results (or view with waveform viewer)
Testing continued
Now we write code to try different inputs
by assigning to connections:
…
initial
begin
#0 s=0; a=0; b=1; expected=0;
#10
a=1; b=0; expected=1;
#10 s=1; a=0; b=1; expected=1;
#10 $stop;
end
Testing continued
• Use $monitor to watch some signals and
see every time they are updated:
…
initial
$monitor(
"select=%b in0=%b in1=%b out=%b
expected out=%b time=%d", s, a, b, f,
expected, $time);
• $time is system function which gives
current (simulated) time
Completed Example
Output
select=0 in0=0 in1=1 out=x, expected out=0 time= 0
select=0 in0=0 in1=1 out=0, expected out=0 time= 2
select=0 in0=1 in1=0 out=0, expected out=1 time= 10
select=0 in0=1 in1=0 out=1, expected out=1 time= 12
select=1 in0=0 in1=0 out=1, expected out=0 time= 20
select=1 in0=0 in1=0 out=0, expected out=0 time= 22
• Expected value (of behavioral Verilog)
matches actual value (of structural
Verilog), so module works for the
inputs patterns tested.
• Simple to extend this testbench to do
exhaustive testing.
Another Testbench for mux2
For more help ...
• Read Verilog Tutorial for many more
ideas on building tests benches,
including:
• more verilog behavioral constructs
 more looping constructs
 use verilog to generate expected output
(rather than enumerate by mimicking
behavior of HW module)
 more output routines
 testing circuits with state
• Read ModelSim manual for use of
waveform viewer
Specifying a clock signal
...
initial
begin
CLK = 1'b0;
forever
#1 CLK = ~CLK;
end
...
• No built in clock in Verilog, so specify one
• Clock CLK above alternates forever in 2 ns
period: 1 ns at 0, 1 ns at 1
Accumulator Example
//Accumulator
module acc (CLK,RST,IN,OUT);
input CLK,RST;
input [3:0] IN;
output [3:0] OUT;
wire [3:0] W0;
add4 myAdd (.S(W0), .A(IN), .B(OUT));
reg4 myReg (.CLK(CLK), .Q(OUT), .D
(W0), .RST(RST));
endmodule // acc
• This module uses prior modules, using wire
to connect output of adder to input of
register
Accumulator TestBench
module accTest;
reg [3:0] IN;
reg CLK, RST;
wire [3:0] OUT;
acc myAcc (.CLK(CLK), .RST(RST),
.IN(IN), .OUT(OUT));
initial
begin
CLK = 1'b0;
repeat (20)
#5 CLK = ~CLK;
end ...
• Clock has a oscillation cycle of _ ns?
Part II
...
initial
begin
#0 RST=1'b1; IN=4'b0001;
#10 RST=1'b0;
end
initial
$monitor("time=%0d: OUT=%1h", $time,OUT);
endmodule // accTest
• What does this initial block do?
• What is output sequence?
• How many lines of output?