On 4.9.20 12.10, druck wrote:
> On 03/09/2020 18:23, A. Dumas wrote:
>> Pancho  wrote:
>>> No mention of pipelines in my copy of "Assembly Language Programming for
>>> the BBC Microcomputer" by Ian Birnbaum.
>>
>> Nor in this seminal machinecode book of the 80's
>> https://ee1.nl/misc/machinecode.pdf (15 MB)
>
> Neither the 6502 or Z80 were pipelined, but if they were it wouldn't be
> something that needs to concern the programmer, even when using
> assembler. The same is true for the ARM.
>
> This isn't the case for all processors, as some early RISC chips such as
> MIPS expose the effects of pipe-lining by having a branch delay slot
> i.e. the instruction after a branch will be executed regardless of
> whether the branch is taken or not, as it is already in the pipeline.
>
> Most other processor flush any instructions in the pipeline after a
> taken branch, which was could be wasteful until good branch prediction
> came along, although most slots were filled with No Ops in practice.
> Processors also have much longer pipelines now, and it would be very
> difficult to code for a variable number of branch delay slots.
>
> ARM tackled pipe-lining issues in in Aarch32 by eliminating the need for
> most small branches by using conditional instructions, which was great
> when programming by hand. This has been deprecated by better branch
> prediction and the use of Thumb, which has an IF THEN ELSE instruction
> to determine which of the following 4 instructions is executed depending
> on a condition.
>
> Aarch64 has eliminated conditional codes from all instructions except
> for branches, but has a small number of conditional arithmetic
> instructions. For example CSEL allows you to calculate the result of
> both the THEN ELSE instructions (usually in parallel with superscalar
> chips) then chose which value to use.
>
> ---druck


In the 32 bit ARM (ARM7TDMI and relatives), the conditionality is in the
top 4 bits of most instructions.

In the same chips, the pipelining peeks in two places:
  - offsets of PC-relative addresses,
  - return addresses of exceptions.

--

-TV

--- SoupGate-Win32 v1.05