4.16 High-Level Languages

• Compiler - program that translate a complete high-level program to a machine language object program
• High level languages usually produce programs that run more slowly and use more memory that machine or assembly language
• High level languages are generally designed to process decimal numbers, and they are much less efficient than assembly language at handling bits
• Machine and assembly language are better suited than high level languages for real time control applications because of their faster execution

4.14 Assembly Laungage

• Programming language that allows the use of mnemonics in place of binary/hex  machine language
• Assember - program that allows computer to convert assembly language to machine language
• Full blown assembler - take a source code file that produce both object code file and list file
• object code file contains the source code 
• list file is a text file that contain source code instruction and any error messages found
• One-line assembler - convert one line of source code at a time into machine language
• Assembly language requires knowledge of
• registers and instruction set of the MPU
• address modes of the MPU
• Assembly languages are not portable - program written for one type of MPU will generally not run on another MPU

4.13 Machine Language

• The language of 0s and 1s
• Only language that computer's circuitry understands
• Lowest level programming language

4.10 The 68HC11 MPU - A Simplified Version

• Buses
• Three external buses and two internal buses
• Three different buses type: 
• address (16 bit, uni-direction)
• data (8 bit, bi-direction)
• control (groupings of all timing and control signals)
• Registers - dedicated memory location inside MPU
• Program counter  - 16-bit counter that controls the sequence in which the instructions are fetched from the memory (hold instruction addresses)
• Data Address Register (DAR) - provides operand address to memory (hold data/operand address)
• Instruction Register - holds the op code while the instruction decoder decodes it and signals the timing and control logic (TCL) to generate proper sequence of control signals (micro-program) to complete the execution of the indicated instruction
• Accumulator and Data Register - hold the operands/data that the ALU operates on during the execution phase of an instruction
• Timing and Control Logic 
• Micro-program is built into MPU's TCL and control how MPU executes a machine language instruction; it cannot be modified by user. 

4.9 Instruction Words

• A program consists of a sequence of binary-coded instructions that the CPU must fetch from memory, decode and execute
• For most computers. instruction words convey two types of info:
• the operation to be performed (op code)
• the address of the operand (data) that is to be operated on (operand address)
• Three basic 68HC11 instruction formats:
• single byte (8 bit op code, no operand address)
• two byte (8 bit op code, 8 bit operand address)
• three byte (8 bit op code, low-order byte operand address, high-order byte operand address)
• Exception: Instructions that use the Y register will have a prebyte precedes the op code byte (indicate internal Y register usage)
• Some instructions will have 2-byte op code: a prebyte and the op code

4.6 Computer Words

• Word is the fundamental unit of information in a computer
• Computer treats each word as a single unit and stores each word in a specific memory location
• Words stored in memory can represent several type of info:
• binary numerical data
• coded data
• instruction codes
• addresses that are part of an instruction

4.5 Microprocessors

• Major differences among MPUs appear in word size, number & types of instructions, type of external control signals and memory.
• Motorola MC68HC11 is a single-chip 8-bits microcontroller that contains an MPU, memory and I/O circuitry.