Assembler - Etudes

Sketches
Alone Coder

   In this section we will publish co-
simple and quick procedures that are recommended
we need to use instead of the ancient and thunder-
zdikh.Small ones will also be included
changes to old procedures that provide any
advantage over the latter.

   A change in the seemingly classic
what procedure DOWN HL, which, as is known,
calculates the byte address on the screen, lying
one pixel below that specified in HL:
      Was: Now:
     INC H INC H
     LD A,H LD A,H
     AND 7 AND 7
     JR NZ,EXIT JR NZ,EXIT;CY=0
     LD A,L LD A,L
     ADD A,32 ADD A,32
     LD L,A LD L,A
     JR C,EXIT JR C,EXIT;CY=1
     LD A,H LD A,H
     SUB 8 ADD A,-8
     LD H,A LD H,A;CY=1
EXIT...EXIT...
   In this version everything remains the same -
mu, except that the CY flag is now
indicates crossing the border of the current
acquaintances. In the UP HL procedure you can produce
make a similar change: instead of ADD A,8
put SUB -8.

   : at the coordinates specified
in registers L=y, E=x, in an arbitrary place
screen (and even behind the screen) in 58(!) clock cycles
The dot appears. Register C is equal to the most significant byte
table addresses ('TABLE), register D=C+2.
     LD H,C
     LD A,(DE);x/8┐
     INC D ├gives low byte
     OR (HL);L(y) ┘INC H
     LD H,(HL);H(y)
     LD L,A
     LD A,(DE);byte(x)
    (X)OR (HL);point setting method
     LD(HL),A
   If instead of the last 2 commands you insert
AND (HL), then this will be the POINT function. To
the procedure worked many times, at the end of it
set DEC D. Tables are generated as follows
following program:
     LD HL,TABLE+256
     LD DE,#4000
GEN0 LD (HL),D ┐
     DEC H │
     LD (HL),E │
     INC H │
     DOWN DE │1st
     LD A,D ├ and
     SUB 88 │2nd
     JR NZ,$+3 │
     LD D,A │
     INC L │
     JR NZ,GEN0 ┘
     INC H
     LD A,128
GEN1 LD (HL),E ┐
     INC H │
     LD (HL),A │
     DEC H │3rd
     RRCA ├ and
     JR NC,$+3 │4th
     INC E │
     INC L │
     JR NZ,GEN1 ┘

   A fragment of a program that allows you to
and the same 19 cycles to execute / not to execute
seven-cycle command (Z - execute, NZ -
do not execute):
     JR NZ,$+4
     JR EXIT
     
EXIT...

   Quickly print 6x8 letters (screen in DE):
     PUSH DE
     LD L,A
     LD B,C
     LD A,2
     RRCA
     DJNZ$-1
     LD (PRN+1),A
     LD H,'FONT
     LD B,8
PRGO PUSH HL
     LD L,(HL)
PRN LD H,1
PR1 ADD HL,HL
     ADD HL,HL
     JR NC,PR1
     LD A,(DE)
     ORH
     LD(DE),A
     INC E
     LD A,L
     LD(DE),A
     DEC E
     INC D
     POP HL
     INC H
     DJNZ PRGO
     POP DE
   If the font stops at 0, then remove LD
B,8,a DJNZ PRGOreplace with JR NZ,PRGO.
   Register C must contain 7,5,3 or 1,
with 7 corresponding to the zero coordinate
letters inside the familiar place (x mod 8=0). Letters in
font 256, and they are all pressed to the left edge
(can be shifted one point to the right). How
you need to change C:
     LD A,C
     SUB 6
     JR NC,$+5
     INC E
     AND 7
     LD C,A

   And finally, I’ll touch on what everyone is already tired of
ZX review calculation of the address of a letter in ROM:
      Was: Now:
     LD L,A LD L,A
     LD H,0 ADD HL,HL
     ADD HL,HL LD H,15
     ADD HL,HL ADD HL,HL
     ADD HL,HL ADD HL,HL
     LD BC,15360
     ADD HL,BC
   We're convinced that you've been bullied all your life -
is it on your ears?