Questions about 8086 processor's instruction times

[Jasper Chow]

**broken link removed**

here is a datasheet of 8086 instruction time. I have two questions now.
First, I don't understand what the 'transfer' in 3rd column stands for.
Besides, for RCR instruction, why reg (1 bit shift) takes 2 cycles, but reg (multi -bit shift) takes 8+4/bit, multiple bit shift can be divided into s string of single bit shifts, but it takes 4 cycles per bit, which is unreasonble for me, and if the mutiple bit shift takes much longer than a series of single bit shifts, what is the point of using mutiple bit shift instruction?

 

[Programmer1971]

While not an answer to your question, I may be able to if no one else pops in. I used to do a bit of ML programming in the 1980s - and have a book that may explain this better in my office. I believe its called the "8080 BUG BOOK" but likely the explanations should apply to the 8086 also. I will take a look monday if you don't get a more knowledgeable answer before then.

Programmer1971 www.oldhackers.com

 

[FvM]

"Transfers" refers to the number of external data memory accesses. An instruction can either perform no external memory access, only an external read or write (1 transfer) or both read and write (2 transfers).

You'll want to analyze real programming problems to understand the advantage of the multi-bit shift operation. As a brief hint, they are mostly used for variable shifts. Coding the same with single bit shifts would involve a larger number of instructions and overall execcution time.

 

[Jasper Chow]

"Transfers" refers to the number of external data memory accesses. An instruction can either perform no external memory access, only an external read or write (1 transfer) or both read and write (2 transfers).

You'll want to analyze real programming problems to understand the advantage of the multi-bit shift operation. As a brief hint, they are mostly used for variable shifts. Coding the same with single bit shifts would involve a larger number of instructions and overall execcution time.

Thanks for your hint. at least I got an idea that using a number of single bit shifts may swell the program, causing storage problem. Mutiple shift is mostly used for variable shifts because variable shifts involve read and write through bus, which slows the processing speed, right?