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Machine Language

Abdi Isai Bochub

2018118785

Pointers in

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HOW?

Pointers are used for constructing data structures.

How are they handled in machine language?

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Suppose one wants to

load a register with the contents of the memory cell that contains the entry on

top of the stack.

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MACHINE LANGUAGE FOR LOADING REGISTERS:

1 R XY - the operand field contains the address of the data to be loaded.
2 RXY - the operand field contains the data to be loaded.

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WHY NOT USE OPCODES 1 & 2?

1

It is unknown what the contents will be.

2

It is not known what the address will be.

3

The address of the top of the stack will vary as the program is executed.

the address of the stack pointer (Data one wants to load).

KNOWN:

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EXPANDING THE MACHINE LANGUAGE TO TAKE ADVANTAGE OF POINTERS

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LOADING A REGISTER FROM A MEMORY CELL THAT IS LOCATED BY MEANS OF A POINTER STORED IN A REGISTER

By using one of the registers as the stack pointer instead of a memory cell, we could reduce this movement of the stack pointer back and forth between registers and memory. But this would mean that we must redesign the load instruction so that it expects the pointer to be in a register rather than in main memory. Thus, instead of the earlier approach, we might define an instruction with op-code D to have the form DR0S, which would mean to load register R with the contents of the memory cell pointed to by register S (Figure above). Then, a complete pop operation could be performed by following this instruction with an instruction (or instructions) to subtract one from the value stored in register S.

Note that a similar instruction is needed to implement a push operation. We might therefore extend the language described in Appendix C further by introducing the op-code E so that an instruction of the form ER0S would mean to store the contents of register R in the memory cell pointed to by register S. Again, to complete the push operation, this instruction would be followed by an instruction (or instructions) to add one to the value in register S.

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ADDRESSING TECHNIQUES

IMMEDIATE ADDRESSING

The operand field contains the data involved explicitly.

DIRECT ADDRESSING

The operand fields contain the address of the data involved.

INDIRECT ADDRESSING

The operand fields of these instructions contain the address of the address of the data.

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Thank You for Watching!

CREDITS:

Computer Science: An Overview 11th Ed. J Glenn Brookshear
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