CDA-4101 Lecture 15 Notes

• microarchitecture are highly variable
• years of tweaking for optimization makes them also more complex to understand
• we will focus on a simple microarchitecture
• although simple, it has the basic characteristics of microarchitectures.
• the purpose of a microarchitecture is to implement an ISA

• the ISA of the microarchitecture we will look at is IJVM
• this is also the ISA that you will be writing programs in
• first we look at the mechanism for implementing the ISA (Chapter 4 of the text)
• then we'll look at the ISA in more detail and write some ISA level programs (Chapter 5 of text)

Java is a high-level objected-oriented language (like C++ but not as cumbersome to program in) A main goal of Java was portability the designers achieved this by defining a virtual machine call the JVM (Java Virtual Machine) you can implement this machine in hardware or software (software being the more typical case) Java programs are compiled into the ISA of this machine any where the machine is implemented you can run the code the JVM really has little to do with object-oriented high-level languages it is a low-level machine specification with a corresponding JVM ISA you could compile other languages down into the JVM ISA code if you wanted (or write JVM ISA code directly)

• to keep things simple, we will only deal with a subset of the full JVM
• in particular, a subset that has only integer instructions
• we will look at a possible hardware implementation of the IJVM rather than a software implementation
• The hardware machine we will discuss is called Mic-1

• this Mic-1 machine will have a microprogram stored in ROM
• the microprogram is responsible for fetching, decoding and executing the individual IJVM instructions
• microprogram has state consisting of registers for helping it implement the ISA
• each IJVM instruction does one simple thing: e.g., add, load from a memory location, etc.
• each IJVM instruction consists of a few fields (usually one or two):
  • first field is opcode (operation code) which specifies what type of instruction, e.g., add, load, branch
  • second field, if it exists, will be the operand, e.g., on memory load, the address to load from

This is the basic hardware that the microprogram will control for each ISA instruction, the microprogram will go through the proper sequence of operations of these hardware elements to accomplish the task access to these hardware elements can only be done at the microprogram level: ISA cannot manipulate this directly Buses A Bus - Only contents of register H can go on this bus and it serves as left input of ALU B Bus - serves as right input to ALU; most registers can put their contents on this bus C Bus - carries ALU output back into registers Registers 32 bit registers each has one or two control inputs for outputting and loading the register overview (details discussed below) MAR - Memory Address register MDR - Memory Data Register PC - Program Counter MBR - Memory Buffer Register SP - Stack Pointer LV - Local Variable pointer CPP - Constant Pool Pointer TOS - Top of Stack register OPC - OpCode register (mostly a scratch register) H - Holding Register is the left ALU input. ALU has 6 control lines and two outputs shifter has 2 control lines not shown are 3 memory control lines total of 29 control lines in microarchitecture