Optron #28 08 июня 1999

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Likbez - Assembler view from afar: Commands unit increments, the direct and relative transition, the organization of the cycle.

       Assembler: a view from afar.


  Continued. Beginning at | | 20, 21, 24, 25

{} Infarh, 1999


      Command of the unit increment


  The very first article of our "Likbekza"
as an example of assembly instructions mentioned
command increments INC. It is time
come to grips with it, as well as her "co-worker" - a team of 
DEC (from the English. "decrement" - multiple decrease in the 
value of the variable on a given constant value). 

  Does not diminish their importance!
After all, if you ever need to increase
or reduce the contents of a register
or the register pair on the unit, then that
do: to work with arithmetic
actions? God forbid!

  And here we come to the aid of these very
team of positive and negative
unit increments. Consider the options
use them, not forgetting about the flag:



      Mnemonic Register F Bits

                       (Flags)


    Operator
     pa-Operand C Z P / V S N H

     tion


     DEC r. x V x 1 x

     DEC (HL)

     DEC (ii + n)

     DEC rr. . . . . .

     DEC ii


     INC r. x V x 0 x

     INC (HL)

     INC (ii + n)

     INC rr. . . . . .

     INC ii



  Legend - similar to previous
Articles "literacy classes."


  Though the team and is very simple, look at an example. 
Before you - a piece of software: 


     LD A, 5

     LD HL, # C000

     LD (HL), A

     INC HL

     DEC A

     LD (HL), A


  The first three lines are clear: in the "A" zapisyvetsya 
number 5 in the register a couple of "HL" - number # C000. 
Then, the transfer the contents of "A" at (HL), then

the contents of bytes at # C000 is five.

  The next line contains the command "INC
HL ", after which the" HL "will
is the number # C001. Accordingly, after
"DEC A" in the battery will no longer five, but four, which is 
the next team to be sent to the address (HL) - # C001. That's 
how you can easily set the increment registers, register pairs 
and byte addressable through the "HL", "IX" or

"IY".



          Extend memories


  From our previous lessons you could
create a false impression that the Z80 processor executes its 
commands sequentially, one after another without any other 
options. 

  But it is not so!

  Of course, follow their chain of command.
But what controls this process?

  Remember the last session - register PC,
he is - the program counter. It saves the address of the team, 
which should performed. Processor, processing a command in the 
memory location addressable by PC, automatically increases the 
his unit. Thus, the contents PC is constantly modified during 
the program. 

  And if we want to change the sequence of commands in the 
program, it suffices to change the contents of PC.

And it can be done by manipulation
bits and check the status of bits and bytes, which is the last 
time with us and discussed. It's time to apply this knowledge 
in practice. So ...




              Direct transition


  There is a command processor, such as JP
(From the English. Jump - jump, jump). For her
should be two-byte parameter: the address to
we want to make the transition. When
command processor will meet JP, he will take
following her address, place it in the PC
and ... move to the next team - the one whose address he had 
just received. 

  Address transition does not necessarily need to be directly 
in the memory trace for JP. With no less success, they may be

the contents of register pair HL, IX, IY. The command is 
written as follows: 


  JP nn

  JP cc, nn

  JP (HL)

  JP (ii)


  - And what is "cc" mean? - Can you
ask.

  And this is - nothing, as a condition. Yes,
transition team can not only be unconditional! Remember the 
table from the past Our lesson: there was a column, as

"Mnemonic conditions." That's it we are here
apply. For example:


  Team Address


  # 6000 JP C, # C003

  # 6003 JP (HL)


  Imagine that in the PC recorded the number of
# 6000 and the processor begins execution
team. Along the way, PC will increase by three
(Exactly as many bytes is the command
JP C, # C003). In the process, will be checked whether the 
condition "C". And it satisfied if CY = 1. After performing

Team Z80 # C003 puts the word in PC and
begins to execute the following command, which is now located at
# C003.

  And if the condition is not satisfied (CY = 0)? Then, the CPU 
is no new data to put the PC will not, and following commands, 
which will serve as Z80, is located on at # 6003. And this team 
is are all the same transition, but now no conditions will not 
be checked, the PC will overwrite the contents of "HL", there 
is will move.


  Referring to the condition "C", for instance, and all obo
others. Here are the mnemonics conditions
You can use the command "JP cc, nn":


       C, NC, Z, NZ, M, P, PE, PO.


  Complex is nothing in them do not. Consider their
all in order:

Z - Zero (Z flag is set)
NZ - not zero (the Z flag is cleared)
C - carry (the flag C is set)
NC - no carry (C flag is cleared)
M - a negative result (flag S mouth
     stalled)
P - positive result (flag S Throw
     shen)
PE - Parity or pereponenie (set

     Flag P / V)
PO - no parity / overflow (flag P / V

     reset)


  Now it remains to draw attention to the
that the conditional can only be a transition
directly to the specified address, and
If the address is in register pair, then
transition can only be unconditional.



          The relative shift


  This transition differs from the direct order
that the team is given no direct address, and
shift, which the processor does not write
in the PC, and adds to it. And in this
case, the displacement can be represented
both positive and negative
number (remember the lesson in "Optocouplers" | 21),
so that the transition can be made in
any direction from the team.

  Recorded a team like this:


  JR e

  JR cc, e


  JR mnemonic comes from the English
"Jump relative", which means "on the transition."

  Note the following command:
"E". It means the amount of displacement with respect to the 
contents of PC. As you can see, and this command can be 
conditional. The rules are deystvut similar to "JP", but here 
is a set of possible conditions somewhat less. Checked only the 
C, NC, Z, NZ. 

  Consider the mechanism of action commands JR
in a conditional form:


  Team Address


  # 6000 JR NC, 2

  # 6002 LD A, 5

  # 6004 .......


  So, Z80 starts to fulfill the team at # 6000. Since it is 
double-byte, PC will be increased by 2 and after command will 
be equal to # 6002. In process the command processor checks the 
condition of NC on the truth. Now, if CY = 0, then to the 
program counter will be added and still command option JR (in 
this case 2), the operation "LD A, 5" will be skipped and the 
next executed command will be the one that is located at # 6004.


  And what is a meaning in such a team?
How it differs from JP? It's very simple. After all, the team 
operates with a JP and is a direct address to the memory of 
three bytes. But sometimes the transition should be made to

short distance (as in our example). Then it makes no sense to 
spend extra bytes for a full address, if enough range 128 ...- 
127. In addition, conditional jump command can be useful when 
writing a so-called relotsiruemyh procedures for which the 
address location in memory do not matter (to these procedures, 
we'll get - all the time).


  By these things add a little question: what
will be, if in our example, instead of "2"
put a "-2"?

  And finally, as usual, on the impact of instruction "JR" and 
"JP" on the flags. So - all These commands abslyutno no effect 
on Flags do not have!




            Looping


  Consider the case where we need to
organize the cyclic execution of some portion of the program. 
Suppose that in the case "B" stores the number of repetitions. 
So ... 


         LD B, 5
 LOOP ........

           body

           cycle

         ........

         DEC B

         JR NZ, LOOP


  The above fragment may initially cause some confusion: "And 
what is LOOP? "The answer is simple. The fact is that when

You write the program in any assembly language, it provides for 
the convenience of such concept as a "mark". All references to 
Ares at her, and not a real address, which is very

facilitates writing and debugging programs.
A word to remember is much simpler than,
for example, "# 7A3E". Yes, and if necessary
add a line or two in a ready-made program for you will not have 
to recalculate all addresses - this will happen automatically.


  I should note that this fragment
Programs like this, you will almost never
encounter. Combination of commands


            DEC B: JR NZ, LOOP

almost always replaced with a single command


                  DJNZ,

reads like a "decrease in and go,
if not zero. "This team - two-byte,
and that its use gives some
saving memory. Again, because of her case in the commonly used 
for organizing counters. It has a disadvantage here: assume

only to 256, but it can be avoided
use nesting.

  Now we rewrite our program to
Given all vysheizozhennogo:


         LD B, 5
 LOOP ........

           body

           cycle

         ........

         DJNZ LOOP


          To be continued ...