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Topic 3 : Controlling the Computer

Topic 3 : Controlling the Computer. More Machine Language & Assembly Language Reference : G & L pp 168 but note that most of the material isn’t in G & L. http://info.comp.lancs.ac.uk. Addressing Modes. In previous examples the instructions have had the address of their operand hard-wired.

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Topic 3 : Controlling the Computer

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  1. Topic 3 : Controlling the Computer More Machine Language & Assembly Language Reference : G & L pp 168 but note that most of the material isn’t in G & L. http://info.comp.lancs.ac.uk

  2. Addressing Modes • In previous examples the instructions have had the address of their operand hard-wired. • Load 5 : 1005 • Store 5 : 2005 • Such a restriction makes it difficult to perform routine tasks such as:- • dealing with constants. • processing arrays. • dealing with complex data structures.

  3. Addressing Modes • Solution is to provide support for common addressing requirements by introducing more powerful addressing modes. • Direct • Immediate • Indexed • Indirect

  4. Direct Addressing • Copies the value stored at the specified address into the accumulator. • Used for accessing stored data. LOAD 10

  5. Immediate Addressing • Copies the operand into the accumulator. • Used for dealing with constants. LOAD #10

  6. Indexed Addressing • Introduces a new register called the index register. • Effective address = the operand field + the value in the index register. • Used for processing arrays LOAD +10

  7. Indirect Addressing • The operand specifies the address of the address of the data. • Used for handling complex data structures. LOAD @10

  8. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  9. Examples ofAddressing Modes ACC : 110 110 210 111 212 Load Immediate 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  10. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 110 Load Direct 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  11. Examples ofAddressing Modes ACC : 210 110 210 111 212 Load Immediate 110 Load Direct 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  12. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 110 Load Direct 110 Load Indexed 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  13. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 110 Load Direct 110 Load Indexed 110 + 2 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  14. Examples ofAddressing Modes ACC : 7 110 210 111 212 Load Immediate 110 Load Direct 110 Load Indexed 110 + 2 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  15. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 110 Load Direct 110 Load Indexed 110 + 2 Load Indirect 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  16. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 110 Load Direct 110 Load Indexed 110 + 2 Load Indirect 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  17. Examples ofAddressing Modes ACC : 10 110 210 111 212 Load Immediate 110 Load Direct 110 Load Indexed 110 + 2 Load Indirect 110 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  18. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  19. Examples ofAddressing Modes ACC : 111 110 210 111 212 Load Immediate 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  20. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 111 Load Direct 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  21. Examples ofAddressing Modes ACC : 212 110 210 111 212 Load Immediate 111 Load Direct 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  22. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 111 Load Direct 111 Load Indexed 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  23. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 111 Load Direct 111 Load Indexed 111 + 2 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  24. Examples ofAddressing Modes ACC : 8 110 210 111 212 Load Immediate 111 Load Direct 111 Load Indexed 111 + 2 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  25. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 111 Load Direct 111 Load Indexed 111 + 2 Load Indirect 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  26. Examples ofAddressing Modes ACC : 0 110 210 111 212 Load Immediate 111 Load Direct 111 Load Indexed 111 + 2 Load Indirect 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  27. Examples ofAddressing Modes ACC : 12 110 210 111 212 Load Immediate 111 Load Direct 111 Load Indexed 111 + 2 Load Indirect 111 112 7 8 113 114 9 ..... 10 210 11 211 12 212 213 13 214 14 Index 2

  28. Relocating Code: The Base Register • If we want to run more than one program at a time we need to be able to relocate programs. • If we can’t then we’d have to write every single program such that they all used different addresses. • Hardware support for relocation can be provided in the form of a base register. • Value in the base register is added to each operand before address resolution.

  29. The Base Register in Action 110 AAA Program A 111 BBB Base Register 112 CCC Load 10 Store 12 100 DDD 113 114 EEE ..... Accumulator Program B FFF 210 GGG 211 Load 10 Store 12 HHH 212 213 III 214 JJJ

  30. The Base Register in Action 110 AAA Program A 111 BBB Base Register 112 CCC Load 10 Store 12 100 DDD 113 114 EEE ..... Accumulator Program B FFF 210 GGG 211 Load 10 Store 12 AAA HHH 212 213 III 214 JJJ

  31. The Base Register in Action 110 AAA Program A 111 BBB Base Register 112 AAA Load 10 Store 12 100 DDD 113 114 EEE ..... Accumulator Program B FFF 210 GGG 211 Load 10 Store 12 AAA HHH 212 213 III 214 JJJ

  32. The Base Register in Action 110 AAA Program A 111 BBB Base Register 112 AAA Load 10 Store 12 200 DDD 113 114 EEE ..... Accumulator Program B FFF 210 GGG 211 Load 10 Store 12 FFF HHH 212 213 III 214 JJJ

  33. The Base Register in Action 110 AAA Program A 111 BBB Base Register 112 AAA Load 10 Store 12 200 DDD 113 114 EEE ..... Accumulator Program B FFF 210 GGG 211 Load 10 Store 12 FFF FFF 212 213 III 214 JJJ

  34. Protecting Programs From Each Other • If multiple programs are running we can check that they stay in their own sections of memory using a limit register. • Each store access is then checked against the limit register to ensure that it is within the programs allocated store.

  35. Multiple Index Registers and Accumulators • We have assumed a single accumulator and a single index register. • Some computers have multiple accumulators and index registers. • We simply need to introduce an extra parameter to the operation to specify which accumulator/index register to use. • In practice many computers don’t distinguish between types of register.

  36. Problems with Machine Code • Large difference between the statement of a typical problem and its solution in machine code. • We have to convert all our operations and addresses into binary or hex patterns. • Every time we change the program we have to recalculate all the jump and data addresses. • Solution: use a higher-level language.

  37. Languages in a Computer System hardware

  38. Languages in a Computer System microprogrammed interpreter hardware

  39. Languages in a Computer System machine code microprogrammed interpreter hardware

  40. Languages in a Computer System Assembly translators machine code microprogrammed interpreter hardware

  41. Languages in a Computer System Systems Assembly translators translators machine code microprogrammed interpreter hardware

  42. Languages in a Computer System General Purpose Systems Assembly translators translators translators machine code microprogrammed interpreter hardware

  43. Assembly Languages • Assembly languages are low-level languages which are very close to machine code. • In general one assembly language instruction = 1 machine code instruction. • Better support for operations: use mnemonics to make writing & understanding code easier. • Better support for data: provides support for writing code without having to hard-wire addresses into the code.

  44. An Assembly Language • Each statement has four fields:- • a label field, a symbol to represent the address of the instruction or data. • an operation field which holds the name of an instruction. • an operand field with optional symbols to denote addressing modes. • a comment field.

  45. An Assembly Language ... contd. label: operation mode operand comment

  46. An Assembly Language ... contd. label: operation mode operand comment here: LDA # 50 ; load 50 into acc there: STA 50 ; store A into loc. 50 JMP there

  47. An Array Processing Program LDA #50 STA COUNT ; store iteration count LDA #0 STI ; set index register to 0 STA TEMP ; initialise sum of elements TOP: LDA TEMP ADD ARRAY+ STA TEMP ; add the next value LDI ADD #1 STI LDA COUNT SUB #1 STA COUNT JNZ TOP

  48. Assessment of Languages Covered • Microcode • easy to build computers to execute microcode. • difficult to program in because instructions do too little. • Machine code • easier to use than microcode. • difficult to write flexible code, programmers must work in hex & instructions still do very little. • Assembly Language • more powerful and flexible then machine code or microcode. • lack of portability and still a long way from the problem.

  49. Summary • Described extensions to our machine code’s addressing scheme to make it more powerful. • Covered the relationship between high-level languages and machine languages/code in the system. • Described a simple assembly language. • Evaluated those languages which we have met so far.

  50. Coming Next Week • A look at how the computer can interact with the outside world. • G & L pp 169-172. • http://info.comp.lancs.ac.uk

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