TUTORIAL #17 – MVUP “Is used to move the main character UP 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 MVUP subroutine.
I will be going through the subroutine MVUP line-by-line and explaining what each line of code is doing
and how it affects the outcome of the program.
467 ***** MOVE UP ******
This is just a comment line to separate each subroutine and
helps to define the start of the MOVE UP subroutine.
468 *
This is just a comment line for use as a line spacer.
469
MVUP LDA CHAR
Line 469 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 469 has a header titled MVUP, which allows it to be
called as a separate subroutine from the main BASIC PROGRAM; “ULTIMA”.
470 CMP #$0B ;HORSE
Once the value of CHAR is loaded into register A, Line 470
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.
471 BEQ
CUMPY2
Line 471 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 CUMPY2. CUMPY2 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 472.
472 CMP #$00 ;SHIP
Line 472 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.
473 BEQ
CUMPY
Line 473 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 CUMPY. CUMPY 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 474.
474 CMP #$01 ;RAFT
Line 474 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.
475 BEQ
CUMPY
Line 475 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. CUMPY 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 476.
476 CMP #$0A
;CART
Line 476 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.
477 BEQ
CUMPY2
Line 477 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 CUMPY2. CUMPY2 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 478.
478 CMP #$0C
;SPEEDER
Line 478 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.
479 BEQ
CUMPY1
Line 479 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. CUMPY1
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 480.
480 CMP #$07
;PLAYER
Line 480 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.
481 BEQ
CUMPY2
Line 481 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 CUMPY2. CUMPY2 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 482.
482 RTS
Line 482 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 MVUP will
be complete and return to the original line statement that called it.
483 CUMPY LDX #$5A
Line 483 is used to “[L]oa[D] register [X]” or
register X with the value equal to 90!
So, you are setting the value of X to equal the HEX value of $5A, which
equals decimal value of 90. NOTE: $5A is
equal to the value of the square to the left of the main character on the
screen. So when you see $5A 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 483 has a header titled CUMPY, which allows it to be
called as a separate SUB-subroutine from the MVUP subroutine”. The CUMPY subroutine is used to check the
collision detection parameters for the SHIP and the RAFT, which only allows
OCEAN movement.
484
LDA MAPSTORE,X
Line 484 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.
485 CMP #$0E
Line 485 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.
486 BEQ
CUMPY3
Line 486 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 CUMPY3. CUMPY3 is the actual part of the MVUP
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 487.
487 RTS
Line 487 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.
488 CUMPY1 LDX #$5A
Line 488 is used to “[L]oa[D] register [X]” or
register X with the value equal to 90!
So, you are setting the value of X to equal the HEX value of $5A, which
equals decimal value of 90.
Also, line 488 has a header titled CUMPY1, which allows it to be
called as a separate SUB-subroutine from the MVUP subroutine”. The CUMPY 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.
489 LDA MAPSTORE,X
Line 489 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.
490 CMP #$0F
Line 490 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.
491 BEQ
UPCNT
Line 491 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 UPCNT. UPCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
492 CMP #$03
Line 492 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.
493 BEQ
UPCNT
Line 493 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 UPCNT. UPCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
494 JMP
CUMPY3
Line 494 tells the program to “[J]u[MP]” to the subroutine CUMPY3. CUMPY3
is the actual part of the MVUP 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.
495 CUMPY2 LDX #$5A
Line 495 is used to “[L]oa[D] register [X]” or
register X with the value equal to 90!
So, you are setting the value of X to equal the HEX value of $5A, which
equals decimal value of 90.
496 LDA MAPSTORE,X
Line 496 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.
497 CMP #$0E
Line 497 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.
498
BEQ UPCNT
Line 498 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 UPCNT. UPCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
499 CMP #$0F
Line 499 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.
500 BEQ
UPCNT
Line 500 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 UPCNT. UPCNT is the direct call to the return
statement, which says there was a collision and the subroutine is returning
back to the main subroutine.
501 CUMPY3 JSR
CLRSET
Line 501 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 501 also has a header name CUMPY3 so it can also be referenced and can be called to run within
the MVUP subroutine. Sub-subroutine CUMPY3 is used to update the variables
that define the new column value. Since
this is the move UP subroutine, we are only worried about the row and not the column
values. The column variable is for LEFT
and RIGHT movement.
502 DEC ROW
Line 502 is used to “[DEC]rement”
whatever value is in ROW and subtract 1 from it!
503 LDA ROW
Line 503 is used to “[L]oa[D] register [A]” or register
A with the value obtained from the variable ROW.
504 STA ROWNO
Line 504 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.
505 LDA COLUMN
Line 505 is used to “[L]oa[D] the [A]ccumulator”
or register A with the value from COLUMN.
506 STA COLUMN1
Line 506 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.
507 JSR
START8
Line 507 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.
508 DEC $FC
Line 508 is used to “[DEC]rement”
the value held within memory location $FC and subtract 1 from it!
509
UPCNT RTS
Line 509 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
MVUP will be complete and return to the original line statement that called
it.
Also, line 509 has a header titled UPCNT, which allows it to be
called as a separate sub-subroutine from the MVUP subroutine.
After the MVUP 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 UP 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 CUMPY3 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.
Joe “kingspud”
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