A certain ISA has a 32-bit word size, uses single word instructions, has 120 opc

ID: 3537569 • Letter: A

Question

A certain ISA has a 32-bit word size, uses single word instructions, has 120 opcodes, 32

registers, and 4Gbyte of byte-addressable memory.

One group of instructions in this ISA takes the form:

OPCODE | DESTINATION REGISTER | SOURCE REGISTER | FLAG | IMMEDIATE VALUE

OPCODE | DESTINATION REGISTER | SOURCE REGISTER 1 | FLAG | SOURCE REGISTER 2

A one-bit flag distinguishes between these two addressing modes.

Another group of instructions takes the form

OPCODE | SOURCE/DESTINATION REGISTER | PC OFFSET

Where PC Offset is the 2%u2019s complement "distance" from the current PC to the labelled location.

1.What is the range of values that can be stored in the %u201CImmediate%u201D field (as a 2%u2019s

complement value, to within +/- 1)?

2. How far (in memory locations) can a labelled location be from an instruction using the PCrelative

addressing mode? (to within +/- 1)

please show equations

Thanks :)

total = 32 bits (word sized instruction)

120 opcodes, so requires 2^8 combinations at least, so 8 bits taken.

32 registers, which is 2^5, so 5 bits taken for register

Equation:

Thus,IMMEDIATE Length = total - opcode length - 2*(register lengths) -flag =32 - 8 - 2*5 -1 = 13 bits.

Answer:

So in 2's complement can represent : -2^13 - 1 to + 2^13 which is -8193 to +8192 in decimal values.

which is roughly +/- 8K (8000)

total =32 bits

120 opcodes, so requires 2^8 combinations at least, so 8 bits taken.

32 registers, which is 2^5, so 5 bits taken for register

Equation:

Thus, PC OFFSET Length = total - opcode length -register length = 32-8-5 = 19 bits.

ANswer:

So in 2's complement can represent : -2^19 - 1 to + 2^19 which is -524289 to +524288 in decimal values (bytes in memory)

which is roughly +/- 512K

Thats the memory range length which can be reached from current PC.

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