TUTORIAL #14 – DRAW

TUTORIAL #14 – DRAW “Is used to help redraw the section of the world map on the screen after each character move of UP/DOWN/RIGHT/LEFT”

I will be going through the subroutine DRAW line-by-line and explaining what each line of code is doing and how it affects the outcome of the program.

 

At this point you in the program you have the world map section of the screen drawn and the program is waiting for you to make a move decision of UP/DOWN/LEFT or RIGHT!  Once this move decision is entered with a key stroke then the program will shift all the graphic tiles/shapes and redraw the display screen with a new section of the world map.

 

360  ********************************

 

This is just a comment line to separate each subroutine and helps to define the start of the DRAW subroutine. 

 

361   DRAW                 LDX   #$00

 

Line 361 is used to “[L]oa[D] register [X]” or register X with the value equal to HEX $00!  So, you are setting the register X to equal the HEX value of $00, which equals decimal value of 0.   This is what is called a zero reset statement used to reset or start a register value that will be used within a loop statement.

 

Also, line 361 has a header titled DRAW, which allows it to be called as a separate subroutine from the START8A subroutine. 

 

362   DRAW1               LDY  BYTEA

 

Line 362 is used to “[L]oa[D] register [Y]” or register Y with the value in BYTEA!  So, you are setting the register Y to equal the value of BYTEA, which was set to zero in the CLRSET subroutine.

 

Also, line 362 has a header titled DRAW1, which allows it to be called as a separate SUB-subroutine within the DRAW subroutine.  In this case it is used within a loop so we don’t have to call the DRAW subroutine and reset the register of X to zero again.

 

363                                LDA   HI,X

 

Line 363 is used to “[L]oa[D] the [A]ccumulator” or register A with the value equal to that held in array HI,X! 

 

364                                STA   HIGH

 

Line 364 is used to “[ST]ore the [A]ccumulator” or take the value that has been stored in register A and place it in variable HIGH.   Again, HIGH is a used to represents the HIGH byte of the memory address location where SHPADR starts.

 

365                                LDA   LO,X

 

Line 365 is used to “[L]oa[D] the [A]ccumulator” or register A with the value equal to that held in array LO,X! 

 

366                                STA   LOW

 

Line 366 is used to “[ST]ore the [A]ccumulator” or take the value that has been stored in register A and place it in variable LOW.   Again, LOW is a used to represents the high byte of the memory address location where SHPADR starts.

 

367                                LDA   TEMP,X

 

Line 367 is used to “[L]oa[D] the [A]ccumulator” or register A with the value equal to that held in array TEMP,X.  

 

368                                STA   (LOW),Y

 

Line 368 is used to “[ST]ore the [A]ccumulator” or take the value that has been stored in register A and place it in array (LOW),Y.   Again, this array will be used to help draw the new transportation mode shape from the TEMP arrays.

 

368                                INY

 

Line 369 is used to “[IN]crement register [Y]” or take the value that has been stored in register Y and add 1 to it.

 

370                                LDA   TEMP1,X

 

Line 370 is used to “[L]oa[D] the [A]ccumulator” or register A with the value equal to that held in array TEMP1,X.  

 

371                                STA   (LOW),Y

 

Line 371 is used to “[ST]ore the [A]ccumulator” or take the value that has been stored in register A and place it in variable (LOW),Y.

 

372                                INX

 

Line 372 is used to “[IN]crement register [X]” or take the value that has been stored in register X and add 1 to it.

 

373                                CPX   #$A0   ;160 LINES

 

Line 373 is used to “[C]om[P]are [X]” what is in register X to the value that is held within #$A0, which is equal to 160.  Remember that 160 equal’s the number of horizontal lines down the display screen that make up all the lines for the shapes/tile graphics.  So this statement is basically doing a comparison to see if the last graphic line has been hit and then start drawing from the top of the screen again.   Also, the “;160 lines” is just a comment to tell you that the value of HEX #$A0 is equal to 160 display screen horizontal graphic lines.

 

374                                BLT   DRAW1

 

Line 374 is used to “[B]ranch on [L]ess [T]han”, which means that as long as the value in register X is less than [<] 160 it will continue to call the subroutine with the header DRAW1.  Once the value of X is greater than [>] 160 and has hit the bottom of the graphics screen, it will bypass line 374 and continue to line 375.  

 

375                                INC   BYTEA

 

Line 375 is used to “[INC]rement” the variable BYTEA by 1 or take the value that has been stored in variable BYTEA and add 1 to it.  Remember that BYTEA stands for the number of bytes in the shape which is two per shape/graphic tile, so by incrementing x2, which the next two statements do, you are side stepping to the right two bytes to get ready to start drawing the next column of shapes down the display screen.  If you didn’t do this double increment, you would end up drawing right on top of the shapes you just had drawn to the screen!

 

376                                INC   BYTEA

 

Line 376 is used to “[INC]rement” the variable BYTEA by 1 or take the value that has been stored in variable BYTEA and add 1 to it.  AGAIN****  Remember that BYTEA stands for the number of bytes in the shape which is two per shape/graphic tile, so by incrementing x2, which the next two statements do, you are side stepping to the right two bytes to get ready to start drawing the next column of shapes down the display screen

 

377                                RTS

 

Line 377 is used to “[R]e[T]urn from [S]ubroutine”, which means that all the actions in this subroutine have been completed and is return to the subroutine that called it, which would actually be the START1A subroutine.

 

So, when subroutine DRAW finishes you should have the following accomplished: 

 

After each time you move your character on the screen the entire display screen must shift the opposite direction to the direction of your movement. So, for example if you want your character to move to the RIGHT, all the graphic tiles/shapes on the display screen will shift [1] position to the LEFT and then be redrawn to the screen.  The DRAW subroutine is used for that purpose.  We can’t use the initial DRAW subroutine because if functions a little different.  The main difference is DRAW is use at the initial display screen draw phase at the start of the program, where DRAW is used after the program has started and draws the tiles after they have been shifted [1] position to the RIGHT/LEFT/UP or DOWN.