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Chapter 7 Low-Level Programming Languages Nell Dale • John Lewis Chapter Goals • List the operations that a computer can perform • Discuss the relationship between levels of abstraction and the determination of concrete algorithm steps • Describe the important features of the Pep/7 virtual machine • Distinguish between immediate mode addressing and direct addressing • Convert a simple algorithm into a machine-language program 7-2 Chapter Goals (cont.) • Distinguish between machine language and assembly language • Describe the steps in creating and running an assemblylanguage program • Convert a simple algorithm into an assembly-language program • Distinguish between instructions to the assembler and instructions to be translated • Describe two approaches to testing • Design and implement a test plan for a simple assemblylanguage program 7-3 Computer Operations • A computer is a programmable electronic device that can store, retrieve, and process data • Data and instructions to manipulate the data are logically the same and can be stored in the same place • Store, retrieve, and process are actions that the computer can perform on data 7-4 Machine Language • Machine language: the instructions built into the hardware of a particular computer • Initially, humans had no choice but to write programs in machine language because other programming languages had not yet been invented 7-5 Machine Language • Every processor type has its own set of specific machine instructions • The relationship between the processor and the instructions it can carry out is completely integrated • Each machine-language instruction does only one very low-level task 7-6 Pep/7: A Virtual Computer • A virtual computer is a hypothetical machine designed to contain the important features of real computers that we want to illustrated • Pep/7 – designed by Stanley Warford – has 32 machine-language instructions • We are only going to examine a few of these instructions 7-7 Features in Pep/7 • The memory unit is made up of 4,096 bytes of storage (4096x8bits) • Word length is 2 bytes (or 16 bits) • Seven registers, four of which we focus on at this point – The program counter (PC) (contains the address of the next instruction to be executed) – The instruction register (IR) (contains a copy of the instruction being executed) – The index register (X register) – The accumulator (A register) 7-8 Features in Pep/7 Figure 7.1 Pep/7’s architecture 7-9 Memory of Pep/7 7-10 Assembly Language • Assembly languages: assign mnemonic letter codes to each machine-language instruction – The programmer uses these letter codes in place of binary digits – A program called an assembler reads each of the instructions in mnemonic form and translates it into the machine-language equivalent 7-11 Figure 7.5 Assembly Process 7-12 Pep/7 Assembly Language Page 208 7-13 Pseudo Operations Useful assembler directives ie. Instructions to assembler Page 209 7-14 Program • Write “Hello” 7-15 Program (immediate addressing) 7-16 Program (direct addressing) 7-17 Program (direct addressing) 7-18 Program (direct addressing + write the initial) 7-19 Features in Pep/7 Figure 7.1 Pep/7’s architecture 7-20 Instruction Format • There are two parts to an instruction – The 8-bit instruction specifier – And optionally, the 16-bit operand specifier Figure 7.2 The Pep/7 instruction format 7-21 Instruction Format • The instruction specifier is made up of several sections – The operation code – The register specifier – The addressing-mode specifier 7-22 Instruction Format • The operation code specifies which instruction is to be carried out 7-23 Instruction Format • The 1-bit register specifier is 0 if register A (the accumulator) is involved in the operation and 1 if register X (the index register) is involved 7-24 Instruction Format • The 2-bit addressing-mode specifier says how to interpret the operand part of the instruction 7-25 Instruction Format Figure 7.3 Difference between immediate-mode and direct-mode addressing 7-26 Machine Code (1/6) LOADA 0007, i LOADX 001F, i 7-27 Machine Code (2/6) LOADA (001F), d LOADX (001F),d 7-28 Machine Code (3/6) STOREA (000A), d ADDA 20A, i 7-29 Machine Code (4/6) ADDX 20A, i ADDA (20A), d 7-30 Machine Code (5/6) ADDX (20A), d CharI (000A) 7-31 Machine Code (6/6) CharO 41,i ‘A’ CharO (000A), d 7-32 A Program Example • Let’s write "Hello" on the screen Page 200 7-33 Hello (1/3) • PC=0 CharO, 48, i Page 201 7-34 Hello (2/3) • PC=3 CharO, 65, i 1 1 Page 201 7-35 Hello (3/3) • PC=0F STOP Page 201 7-36 Code 7-37 Pep/7 Simulator • A program that behaves just like the Pep/7 virtual machine behaves • To run a program, we enter the hexadecimal code, byte by byte with blanks between each Page 202 7-38 Another Program • Using direct addressing method – 11100000 => 11100001 • Insert data after the program code • Correction (p. 204) – Line #6 : • 10 (16 decimal) – Line #7 : • 11 (17 decimal) 7-39 Enhanced Version • Refer the table on the textbook (page 205) 7-40 A New Program Page 213 7-41 Pseudocode Page 213 7-42 Completed Program Page 214 Our Completed Program Page 215 Testing • Test plan: a document that specifies how many times and with what data the program must be run in order to thoroughly test the program • A code-coverage approach designs test cases to ensure that each statement in the program is executed • Data-coverage testing is another approach; it designs test cases to ensure that the limits of the allowable data are covered 7-45 Ethical Issues: Software Piracy, Copyrighting • Research indicated that, globally, 11.5 billion dollars were lost in the year 2000 to pirated software • Advocates of open-source code believe that a program’s original source code should be in the public domain • Respecting the copyrights of software, if it is not open code, is important from a number of perspectives 7-46