TUTORIAL #18 – MVDOWN “Is used to move the main character DOWN on a key press, plus follow all collision detection requirements per character shape used”.
All the movement within Ultima 1 is based on “key press”
functions because this game was created way before the mouse! In the original Ultima 1 you have four keys
representing the North, South, East, and West movement, which consisted of the
following keys: [ENTER],[ /], [ ; ]
, [’ ].
Based on the represented shape on the screen ie. Fighter, horse, cart,
etc… MVUP will also perform the required collision detections of the specific
terrain on the screen based on the type of shape used. Each shape has a different type of collision
detection so for example; the horse can travel over plains, trees, but not
ocean or mountains. The ship and raft
can travel over ocean tiles but not any other tiles. The land speeder can travel over plains but
not trees or mountains. Etc… These types of collision detections are
performed within the MVDOWN subroutine.
I will be going through the subroutine MVDOWN line-by-line and explaining what each line of code is doing
and how it affects the outcome of the program.
510 ***** MOVE DOWN ******
This is just a comment line to separate each subroutine and
helps to define the start of the MOVE DOWN subroutine.
511 *
This is just a comment line for use as a line spacer.
512
MVDOWN LDA CHAR
Line 512 is used to “[L]oa[D] register [A]” or
register A with the value in the variable CHAR!
So, you are setting the register A to equal the value which was initial
setup in the subroutine INITIAL. As the
transportation mode changes so does the CHAR value. So when a numeric key from 1 thru 6 is
pressed the value of CHAR get updated and modified.
Also, line 512 has a header titled MVDOWN, which allows it to be
called as a separate subroutine from the main BASIC PROGRAM; “ULTIMA”.
513 CMP #$0B ;HORSE
Once the value of CHAR is loaded into register A, Line 513
is used to “[C]o[MP]are”
what is in register A to the HEX value #$0B or decimal value of 11! The value in CHAR is the number representing
the shape/tile graphic for the main character shown on the computer screen. Each character or graphic tile has a numeric
value constant with the shape in the shape database. So as CHAR changes it stores a new value of
the main character shape/graphic tile such as fighter, horse, cart, ship,
etc... The [ ; ] symbol is a remark
symbol that tell the compiler to ignore everything after the ;. The HORSE reference tells anyone reading the
code that #$0B is a value that represents the HORSE shape.
514 BEQ
DUMPY2
Line 514 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$0B and it equals this value, in this case
11, then the program will branch or call jump to subroutine DUMPY2. DUMPY2 is the collision detection part of
the subroutine for the HORSE, which decides if it collides with specific
terrain types and if the collision is legal.
Otherwise the program moves on to the next code line 515.
515 CMP #$00 ;SHIP
Line 515 is used to “[C]o[MP]are” what is in register A to the HEX value $00 or
the decimal value of 0! The value in
CHAR will represent the shape/tile graphic for the SHIP in the middle of the
screen. The [ ; ] symbol is a remark
symbol that tell the compiler to ignore everything after the ;. The SHIP reference tells anyone reading the
code that #$00 is a value that represents the SHIP shape.
516 BEQ
DUMPY
Line 516 is used to “[B]ranch
on Result Zero Z=1”, which means that if the value in register A is compared to
#$00 and it equals this value, in this case 0, then the program will branch or
call jump to subroutine DUMPY. DUMPY is the collision detection part of the
subroutine for the SHIP, which decides if it collides with specific terrain
types and if the collision is legal.
Otherwise the program moves on to the next code line 517.
517 CMP #$01 ;RAFT
Line 517 is used to “[C]o[MP]are” what is in register A to the HEX value $01 or
the decimal value of 1! The value in
CHAR will represent the shape/tile graphic for the RAFT in the middle of the
screen. The [ ; ] symbol is a remark
symbol that tell the compiler to ignore everything after the ;. The RAFT reference tells anyone reading the
code that #$01 is a value that represents the RAFT shape.
518 BEQ
DUMPY
Line 518 is used to “[B]ranch
on Result Zero Z=1”, which means that if the value in register A is compared to
#$01 and it equals this value, in this case 1, then the program will branch or
call jump to subroutine RAFT1. DUMPY is
also the collision detection part of the subroutine for the RAFT, which decides
if it collides with specific terrain types and if the collision is legal. Otherwise the program moves on to the next
code line 519.
519
CMP #$0A ;CART
Line 519 is used to “[C]o[MP]are” what is in register A to the HEX value $0A or
the decimal value of 10! The value in
CHAR will represent the shape/tile graphic for the CART in the middle of the
screen. The [ ; ] symbol is a remark
symbol that tell the compiler to ignore everything after the ;. The CART reference tells anyone reading the
code that #$0A is a value that represents the CART shape.
520 BEQ
DUMPY2
Line 520 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$0A and it equals this value, in this case
10, then the program will branch or call jump to subroutine DUMPY2. DUMPY2 is the collision detection part of
the subroutine for the CART, which decides if it collides with specific terrain
types and if the collision is legal.
Otherwise the program moves on to the next code line 521.
521 CMP #$0C
;SPEEDER
Line 521 is used to “[C]o[MP]are” what is in register A to the HEX value $0C or
the decimal value of 12! The value in
CHAR will represent the shape/tile graphic for the LAND SPEEDER in the middle
of the screen. The [ ; ] symbol is a
remark symbol that tell the compiler to ignore everything after the ;. The LAND SPEEDER reference tells anyone
reading the code that #$0C is a value that represents the SPEEDER shape.
522 BEQ
DUMPY1
Line 522 is used to “[B]ranch
on Result Zero Z=1”, which means that if the value in register A is compared to
#$0C and it equals this value, in this case 12, then the program will branch or
call jump to subroutine LANDSPR. DUMPY1
is the collision detection part of the subroutine for the LANDSPR, which
decides if it collides with specific terrain types and if the collision is
legal. Otherwise the program moves on to
the next code line 523.
523 CMP #$07
;PLAYER
Line 523 is used to “[C]o[MP]are” what is in register A to the HEX value $07 or
the decimal value of 7! The value in
CHAR will represent the shape/tile graphic for the main FIGHTER character in
the middle of the screen. The [ ; ]
symbol is a remark symbol that tell the compiler to ignore everything after the
;. The PLAYER reference tells anyone
reading the code that #$07 is a value that represents the PLAYER shape.
524 BEQ
DUMPY2
Line 524 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$07 and it equals this value, in this case
7, then the program will branch or call jump to subroutine DUMPY2. DUMPY2 is also the collision detection part
of the subroutine for the PLAYER, which decides if it collides with specific
terrain types and if the collision is legal.
Otherwise the program moves on to the next code line 525.
525 RTS
Line 525 is used to “[R]e[T]urn from [S]ubroutine”,
which means that after the SUB-subroutine of each character tile shape
reference is called and updated, it returns back and then subroutine MVDOWN
will be complete and return to the original line statement that called it.
526 DUMPY LDX #$82
Line 526 is used to “[L]oa[D] register [X]” or register
X with the value equal to 130! So, you
are setting the value of X to equal the HEX value of $82, which equals decimal
value of 130. NOTE: $82 is equal to the
value of the square to the bottom of the main character on the screen. So when you see $82 it is talking about one
of four squares surrounding the main character on the screen. Each of the movement subroutines does this
check to the corresponding square next to the character tile on the screen.
Also, line 526 has a header titled DUMPY, which allows it to be
called as a separate SUB-subroutine from the MVDOWN subroutine”. The DUMPY subroutine is used to check the
collision detection parameters for the SHIP and the RAFT, which only allows
OCEAN movement.
527
LDA MAPSTORE,X
Line 527 is used to “[L]oa[D] register [A]” or
register A with the value in the array MAPSTORE,X and stored it in
register A.
528 CMP #$0E
Line 528 is used to “[C]o[MP]are” what is in register A to the HEX value $0E or
the decimal value of 14! This value is
what is equal to the OCEAN graphic tile/shape and if the comparison matches
then it is a valid movement and continues with the redraw of the screen. If the comparison does not match then it is a
collision and the subroutine returns back to the main subroutine.
529 BEQ
DUMPY3
Line 529 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$0E and it equals this value, in this case
14, then the program will branch or call jump to subroutine DUMPY3. DUMPY3 is the actual part of the MVDOWN
subroutine that when called has passed all the collision detection and draws
the new screen display of the new location for the character shape on the world
map. Otherwise the program moves on to
the next code line 530.
530 RTS
Line 530 is used to “[R]e[T]urn from [S]ubroutine”,
which means that the collision detection did not pass for the movement
direction so the subroutine is returned and awaits another movement command.
531 DUMPY1 LDX #$82
Line 531 is used to “[L]oa[D] register [X]” or
register X with the value equal to 130!
So, you are setting the value of X to equal the HEX value of $82, which
equals decimal value of 130.
Also, line 531 has a header titled DUMPY1, which allows it to be
called as a separate SUB-subroutine from the MVDOWN subroutine”. The DUMPY subroutine is used to check the
collision detection parameters for the LAND SPEEDER, which allows for OCEAN and
PLAIN movement but not through the TREES or MOUNTAINS.
532 LDA MAPSTORE,X
Line 532 is used to “[L]oa[D] register [A]” or
register A with the value in the array MAPSTORE,X and stored it in
register A.
533 CMP #$0F
Line 533 is used to “[C]o[MP]are” what is in register A to the HEX value $0F or
the decimal value of 15! The value of 15
is equal to the MOUNTAIN graphic tile and this is a collision check to see if
the move will cause the character to move on top of the MOUNTAIN shape on the
world map.
534 BEQ
DNCNT
Line 534 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$0F and it equals this value, in this case
15, then the program will branch or call jump to subroutine DNCNT. DNCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
535 CMP #$03
Line 535 is used to “[C]o[MP]are” what is in register A to the HEX value $03 or
the decimal value of 3! The value 3 is
equal to the graphic tile/shape of the TREE.
536 BEQ
DNCNT
Line 536 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$03 and it equals this value, in this case
3, then the program will branch or call jump to subroutine DNCNT. DNCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
537 JMP
DUMPY3
Line 537 tells the program to “[J]u[MP]” to the subroutine DUMPY3. DUMPY3
is the actual part of the MVDOWN subroutine that when called has passed all the
collision detection and draws the new screen display of the new location for
the character shape on the world map.
538 DUMPY2 LDX #$82
Line 538 is used to “[L]oa[D] register [X]” or
register X with the value equal to 130!
So, you are setting the value of X to equal the HEX value of $82, which
equals decimal value of 130.
539 LDA MAPSTORE,X
Line 539 is used to “[L]oa[D] register [A]” or
register A with the value in the array MAPSTORE,X and stored it in
register A.
540 CMP #$0E
Line 540 is used to “[C]o[MP]are” what is in register A to the HEX value $0E or
the decimal value of 14! The value 14 is
equal to the graphic tile/shape of the OCEAN.
541
BEQ DNCNT
Line 541 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$14 and it equals this value, in this case
14, then the program will branch or call jump to subroutine DNCNT. DNCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
542 CMP #$0F
Line 542 is used to “[C]o[MP]are” what is in register A to the HEX value $0F or
the decimal value of 15! The value 15 is
equal to the graphic tile/shape of the MOUNTAIN.
543 BEQ
DNCNT
Line 543 is used to “[B]ranch
on Result Zero Z[=]1”, which means that if the
value in register A is compared to #$15 and it equals this value, in this case
15, then the program will branch or call jump to subroutine DNCNT. DNCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
544 DUMPY3 JSR
CLRSET
Line 544 tells the program to “[J]ump
to the [S]ub[R]outine”
with the header CLRSET. The subroutine
CLRSET takes the specific variables used within the DRAW subroutine and clears
them to zero when the screen is redrawn for the character movement.
Line 544 also has a header name DUMPY3 so it can also be referenced and can be called to run within
the MVDOWN subroutine. Sub-subroutine DUMPY3 is used to update the variables
that define the new column value. Since
this is the move DOWN subroutine, we are only worried about the row and not the
column values. The column variable is
for LEFT and RIGHT movement.
545
INC ROW
Line 545 is used to “[INC]rement”
whatever value is in ROW and add 1 to it!
546 LDA ROW
Line 546 is used to “[L]oa[D] register [A]” or register
A with the value obtained from the variable ROW.
547 STA ROWNO
Line 547 is used to “[ST]ore the [A]ccumulator”
which has the value obtained from ROW.
ROWNO will now be used as a temporary ROW variable that can be modified
without disturbing the original ROW value.
548 LDA COLUMN
Line 548 is used to “[L]oa[D] the [A]ccumulator”
or register A with the value from COLUMN.
549 STA COLUMN1
Line 549 is used to “[ST]ore the [A]ccumulator”
which has the value obtained from COLUMN.
COLUMN1 will now be used as a temporary COLUMN variable that can be
modified without disturbing the original COLUMN value.
550 JSR
START8
Line 550 tells the program to “[J]ump
to the [S]ub[R]outine”
with the header START8. The subroutine
START8 is where the process of redrawing the screen takes place after the
movement process has occurred.
551 INC $FC
Line 551 is used to “[INC]rement”
the value held within memory location $FC and add 1 to it!
552
DNCNT RTS
Line 552 is used to “[R]e[T]urn from [S]ubroutine”,
which means that after the SUB-subroutine of each character tile shape
reference is called and updated, it returns back to here and then subroutine
MVDOWN will be complete and return to the original line statement that called
it.
Also, line 552 has a header titled DNCNT, which allows it to be
called as a separate sub-subroutine from the MVDOWN subroutine.
After the MVDOWN subroutine completes you should have accomplished
the following:
Depending on what type of shape/graphic tile you had as your main
character shape, you will have pressed the enter key and wanted to move your
character DOWN one space on the world map.
To be able to do this the subroutine will check what type of character
shape is being used, then check to see if the terrain you are about to move
into is non-collision type terrain. The
subroutine check the value of the terrain shape against the legal movement
values and if it is ok it continue on to the Sub-subroutine DUMPY3 that
modifies the variable and returns out of the subroutine getting ready to draw
the new world map screen display. If
there is a collision detected against a terrain type that is not legal, then
the subroutine just returns out of the main subroutine and waits for a new
movement key press.
As always, please email me any questions you might have.
Ultima_revisited@yahoo.com
Joe “kingspud”
