A working computer Y86 CPU features March 30, 2020 Patrice - - PowerPoint PPT Presentation

A working computer

Y86 CPU features

March 30, 2020 Patrice Belleville / Geoffrey Tien 1

A working computer

• Von Neumann architecture:

March 30, 2020 Patrice Belleville / Geoffrey Tien 2

Memory (contains program code and data) CPU (central processing unit) Control unit Arithmetic & logic unit Input/output

A working computer

• Memory

– Each memory location (address) contains a fixed number of bits – Most commonly this number is 8 – Values that use more than 8 bits are stored in multiple consecutive memory locations

• Characters use 8 bits (ASCII) or 16/32 (Unicode)
• Integers use 32 or 64 bits
• Floating point numbers use 32, 64, or 80 bits

– Larger multi-byte values may need to be retrieved from memory over several clock cycles

March 30, 2020 Patrice Belleville / Geoffrey Tien 3

A working computer

• Arithmetic & logic unit

– A combinational component which performs arithmetic and logical

• perations

– e.g. +, –, *, /, AND, OR, etc. – The function is chosen by multiplexers, select bits will be given by the control unit

March 30, 2020 Patrice Belleville / Geoffrey Tien 4

A working computer

• Assume that we already have functional circuit blocks to

perform various operations such as AND, NOT, etc.

– 𝑜-bit inputs 𝐵, 𝐶 1-bit input 𝑑_𝑗𝑜 3-bit select input – 𝑜-bit output 𝑠𝑓𝑡 – each block can be represented as a subsystem with 𝑜-bit inputs for 𝐵, 𝐶, and a 𝑜-bit output – Using multiplexers, these functional blocks can be arranged in a circuit diagram to implement an ALU with some function select table, e.g.

March 30, 2020 Patrice Belleville / Geoffrey Tien 5

Arithmetic & logic unit

𝑡𝑓𝑚 Function 𝑡𝑓𝑚 Function 0 0 0 𝑠𝑓𝑡 ← 𝐵 + 𝑑_𝑗𝑜 1 0 0 𝑠𝑓𝑡 ← 0…0 & 𝑑_𝑗𝑜 (𝑜 bits) 0 0 1 𝑠𝑓𝑡 ← 𝐵 + 𝐶 + 𝑑_𝑗𝑜 1 0 1 𝑠𝑓𝑡 ← 𝐶 + 𝑑_𝑗𝑜 0 1 0 𝑠𝑓𝑡 ← 𝐵 + ~𝐶 + 𝑑_𝑗𝑜 1 1 0 𝑠𝑓𝑡 ← ~𝐶 + 𝑑_𝑗𝑜 0 1 1 𝑠𝑓𝑡 ← 𝐵 and 𝐶 1 1 1 𝑠𝑓𝑡 ← 𝐵 or 𝐶 Exercise: Try to design this! Find common things that happen for specific select bits

A working computer

• Control unit

– A sequential device that controls the process of:

• reading instructions and data from memory
• deciding which instructions to execute
• executing the instructions sequentially

– not quite true, but this is how it appears to the user

March 30, 2020 Patrice Belleville / Geoffrey Tien 6

A working computer

• Our working computer:

– Implements the (fictional) Y86 architecture presented in the textbook by Bryant and O'Hallaron, used in CPSC 213 and 313 – A small subset of the IA32 (Intel 32-bit) architecture

• The Y86 CPU has:

– 12 types of instructions – One program counter (PC) register

• contains the address of the next instruction to execute

– 8 general-purpose 32-bit registers

• used for temporary values we are currently working on

– One 48-bit instruction register (IR)

• used to hold pieces of an instruction while it gets retrieved from memory

– Condition code register (CCR)

• holds condition flags for the results of arithmetic operations

March 30, 2020 Patrice Belleville / Geoffrey Tien 7

A working computer

March 30, 2020 Patrice Belleville / Geoffrey Tien 8

Partial Y86 execution path (not guaranteed to be accurate!)

A working computer

• Example instruction 1: irmovl 0x1A, %ecx

– This instruction stores a constant value in a register – In this case, the value 1A (hexadecimal) is stored in %ecx – "immediate-register move 0x1A to %ecx" – The value 0x1A is "immediately" available as part of the binary instruction, and does not need to be retrieved from memory or a register

• Example instruction 2: subl %eax, %ebx

– The subl instruction subtracts it arguments – %eax and %ebx refer to source and source/destination registers – This instruction takes the value contained in %eax, subtracts it from the value contained in %ebx, and stores the result back in %ebx

March 30, 2020 Patrice Belleville / Geoffrey Tien 9

Y86 instructions

A working computer

• Sample program:

irmovl 0x3, %eax irmovl 0x35, %ebx subl %eax, %ebx halt

March 30, 2020 Patrice Belleville / Geoffrey Tien 10

Y86 instructions

A working computer

• How does the computer know which instruction does what?

– Each instruction is a sequence of 8 to 48 bits – The first 8 bits (icode/fcode) determine the instruction type

• Control unit uses these bits to decide how many more bits need to be

retrieved from memory to assemble the complete instruction

• Also will allow the control unit to send the appropriate select signals to the

various multiplexers in the CPU

March 30, 2020 Patrice Belleville / Geoffrey Tien 11

Y86 instructions