World: r3wp

but the instructions which operate directly in memory are rarely 
used, because they consume more time.

More time than the combination of a load to a register and an add 
from that register?

yes

depending what operations follows , but right the most time

Wow. It's the same amount of work, in fewer instructions, and it 
takes more time. How does that happen? Microcode?

the reason is that most register operations use one opcode ob one 
byte length

*of one byte length

operation into memory use an opcode of 3 to 6  byte length

so in 1 operation into memory , we can perform several operation 
with registers

*so during 1 operation

How many bytes in a load from memory? I suppose on the addressing 
mode...

3

*it depends on the addressing mode

yep, you're right

3 is the minimum

one for the operation opcode and 2 for the address on 16 bits

but in fact it can be occured in one opcode of  length 1, if the 
adress is already contained in one 16 bit register .

that's mostly the case

so, the correct answer is: 1

What happens if you EXT8 a value that already has data in the higher 
bytes? I'm guessing it will just overwrite that data...

wrong

What happens? I don't have rebcode here to test.

I'm going off docs and memory here.

ext8 (128 + 65536) = -128 instead of 128

fooooollll

no it works

ext 128 = -128

I'm thinking that you may be able to efficiently store your 8-bit 
registers in your 16-bit registers and break them out when you need 
them.

Internally, they would really be 32-bit registers, of course.

Is the accumulator internally 8 or 16-bit?

8

it is A

_a in my source

What do 16-bit arithmetic instructions use for an accumulator?

HL

but most of instructions act on A

Is 8-bit or 16-bit code more common?

8 bit

because it's more compact and faster

and many operations occur only on 8 bit registers

like rotation operations

and bit manipulations

but the first reason is that 8bit register operations are faster

Well, there is no reason to change that balance - the code expects 
it. 16-bit operations can be sped up using a few tricks if necessary, 
like temporary registers that are used internally. You can even name 
those registers _bc, _de and _hl if you like :)

Do the rotation instructions affect the carry flag?

some of them, it's an option

the carry flag can be injected to the left or to the right

or not

there are 12 different rotations

Z80 is good with rotation