Why IBM 360 Model 65 and 75, staggered in two separate main memory units

IN IBM 360 model 65 and 75 address are staggered in two seperate main memory units( all even number words in one unit and all odd number words in another unit)What might be the purpose of this technique?

Answers:

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The memory is divided into two blocks, even and odd blocks called low order and high order blocks with odd addresses is termed as low order and the blocks with even addresses is referred as high order blocks. In odd it stores arithmetic and will come in single word and in even it stores floating point numbers and will come in two words. Also to maintain the old architecture because the old one used 8-bit register and this new one use 16-bit of 8,8 bit each.Suppose you want to read a word from location 125. Okay, the L.O. byte of the word comes from location 125 and the H.O. word comes from location 126.
The advantage of using even and odd registers is that we can access up to two words. This increase the speed and efficiency of the whole system.
Think of memory as a linear array of bytes. The address of the first byte is zero and the address of the last byte is (2**n)-1.
To execute the equivalent of the Pascal statement "Memory [125]:= 0;" the CPU places the value zero on the data bus, the address 125 on the address bus, and asserts the write line (since the CPU is writing data to memory:

The above discussion applies only when accessing a single byte in memory. So what happens when the processor accesses a word or a double word? Since memory consists of an array of bytes, how can we possibly deal with values larger than eight bits?



The 80x86 family deals with this problem by storing the L.O. byte of a word at the address specified and the H.O. byte at the next location.
The IBM Models 65 & 75 used two separate registers namely odd register & even register for odd-numbered words & even-numbered words respectively. The actual idea behind the application of this technique was that the integers at that time required only one word & hence rightly labeled as odd-numbered word, while floating points required two words & hence named as even-numbered words. Thus the two types of values were stored in two different registers as mentioned above.
The following were the major advantages: -
• Memory was greatly saved by allotting almost exactly the required space. At that time, memory was very limited & costly.
• Accessing the address was much faster mentioning the address was not required. Because on finding the integer value it will automatically go to odd register & on encountering floating point value, it will automatically go to even register.
• Parallel accessing the two registers was possible by a single clock. So processing speed was increased.