What's the addr range for each memory module in this system?

[student2005]

4K x 8 RAM memory is connected to the CPU.

If the logic Gate-1 is a OR gate, what is the address range for each memory chip (i.e. RAM-0, RAM-1, RAM-2, and RAM-3)?

If the logic Gate-1 is a NOR gate, what is the address range for each memory chip (i.e. RAM-0, RAM-1, RAM-2, and RAM-3)?

Thanks.


The Memory System

74LS138 Truth Table

 

[eltonjohn]

Re: What's the addr range for each memory module in this sys

These memories are mapped in the last 4K segment of a 64k addressing space
Because each one is 1k long they are mapped at addresses
RAM 0 -0xF000~0xF3FF
RAM 1- 0xF400~0xF7FF
RAM 2- 0xF800~0xFBFF
RAM 3 -0XFC00~0xFFFF

I hope this is NOT A HOMEWORK .That you are supposed to solve ON YOUR OWN!

 

[student2005]

Re: What's the addr range for each memory module in this sys

These memories are mapped in the last 4K segment of a 64k addressing space
Because each one is 1k long they are mapped at addresses
RAM 0 -0xF000~0xF3FF
RAM 1- 0xF400~0xF7FF
RAM 2- 0xF800~0xFBFF
RAM 3 -0XFC00~0xFFFF

I hope this is NOT A HOMEWORK .That you are supposed to solve ON YOUR OWN!

Is your answer derived from the fact that Gate-1 is a NOR gate? Or, OR gate?

The RAMs are connected to 74LS138 outputs (i.e. 0, 1, 2, and 3). From the 74LS138 truth table, the input to C of 74LS138 must be LOW. If Gate-1 is NOR, A15~A12 CANNOT be 0000. But it can be 0001, 0011, 0111, 1111, etc. So, why do you think it must be 1111 or F, and mapped to last 4K of the 64K addressing space? Pls advise.

 

[echo47]

Re: What's the addr range for each memory module in this sys

With an OR gate, bit A12 through A15 must all be off:
RAM-0: 0000-03FF
RAM-1: 0400-07FF
RAM-2: 0800-0BFF
RAM-3: 0C00-0FFF

With a NOR gate, any bit A12 through A15 can be on, so the RAMs appear at multiple addresses:
RAM-0: 1000-13FF, 2000-23FF, 3000-33FF, ... , F000-F3FF
RAM-1: 1400-17FF, 2400-27FF, 3400-37FF, ... , F400-F7FF
RAM-2: 1800-1BFF, 2800-2BFF, 3800-3BFF, ... , F800-FBFF
RAM-3: 1C00-1FFF, 2C00-2FFF, 3C00-3FFF, ... , FC00-FFFF

 

[student2005]

Re: What's the addr range for each memory module in this sys

With an OR gate:
RAM-0: 0000-03FF
RAM-1: 0400-07FF
RAM-2: 0800-0BFF
RAM-3: 0C00-0FFF

With a NOR gate, the RAMs appear at multiple addresses:
RAM-0: 1000-13FF, 2000-23FF, 3000-33FF, ... , F000-F3FF
RAM-1: 1400-17FF, 2400-27FF, 3400-37FF, ... , F400-F7FF
RAM-2: 1800-1BFF, 2800-2BFF, 3800-3BFF, ... , F800-FBFF
RAM-3: 1C00-1FFF, 2C00-2FFF, 3C00-3FFF, ... , FC00-FFFF

I agree.

So, it's useful to have such a system (with NOR gate) that the RAMs appear at multiple addresses? Thanks.

 

[echo47]

In most systems, there is no advantage to having a device appear at multiple addresses, except that you may be able to eliminate some hardware by not bothering to decode the upper address bits. A few system use multiple addresses deliberately to map certain regions into different numbers of wait states, or other special purposes.

 

[eltonjohn]

Re: What's the addr range for each memory module in this sys

oh i'm sorry i didn't read the OR ,NOR specs ....i assumed it was an AND my fault !!