{"id":43,"date":"2019-10-25T07:50:00","date_gmt":"2019-10-25T06:50:00","guid":{"rendered":"https:\/\/techblog.dansbo.dk\/?p=43"},"modified":"2025-05-17T13:49:36","modified_gmt":"2025-05-17T12:49:36","slug":"my-1st-assembler-game-for-cx16-part-2","status":"publish","type":"post","link":"https:\/\/techblog.dansbo.dk\/?p=43","title":{"rendered":"My 1st assembler game for CX16 \u2013 part 2"},"content":{"rendered":"\n

Continuing from part 1<\/a> where I created 3 small functions: GotoXY, HLine and VLine. I am going to create a couple more \u201chelper\u201d functions as well as the function that will initialize the screen and make it ready to start the game.<\/p>\n\n\n\n

First I will look at the function to print a string on the screen. It is actually fairly simple. We start by finding the address in memory where the string is stored. When that is found, we print the character that is stored there, increment the address and start over until we reach a 0-byte. As the function being used (CHROUT) automatically move the cursor we do not have to think about anything other than incrementing the address and checking for 0-byte.<\/p>\n\n\n\n

; **************************************************************\n; PrintStr\n; **************************************************************\n; Description:\tWrite a zero-terminated PETSCII string to scr\n; **************************************************************\n; Inputs:\tRegister X = Low byte of string start address\n;\t\tRegister Y = High byte of string start address\n; **************************************************************\nPrintStr:\n\tstx<\/font>\tTMP0\t\t; Store string start address in\n\tsty<\/font>\tTMP1\t\t; Zero-page memory in order to\n\t\t\t\t; do indirect addressing\n\tldy<\/font>\t#0<\/font>\t\t; Y used as offset, start at 0\n.doprint\n\tlda<\/font>\t(TMP0), Y\t; Load character into reg A\n\tbeq<\/font>\t.printdone\t; If char=0, we are done\n\tjsr<\/font>\tCHROUT\t\t; Print the character\n\tiny<\/font>\t\t\t; Increment offset (Y)\n\tjmp<\/font>\t.doprint\t; Loop back, get next character\n.printdone:\n\trts<\/font><\/code><\/pre>\n\n\n\n
Now that we have the PrintStr function, let me show how it is called. I will create a small snippet of code to show how to use the PrintStr function. This snippet will not function correctly on its own, but if it is combined with the information found in part 1, it should be possible to assemble a .prg file that will actually run on the Commander X16.<\/p>\n\n\n\n
; Load X with low-byte of .title address\n\tldx<\/font>\t#<.title\n\t; Load Y with high-byte of .title address\n\tldy<\/font>\t#>.title\n\tjsr<\/font>\tPrintStr\n\n; Variable containing our string\n.title\t!pet\t\"x16-maze\"<\/font>,0<\/font><\/code><\/pre>\n\n\n\n
The next function converts a !byte value into a 3 digit PETSCII string. As we all know, a byte can hold any value from 0 to 255 so we need 3 digits to represent any number that can be held in a byte. The function works directly on 2 global variables. A !byte called .lvl and a !pet called .lvltxt<\/p>\n\n\n\n
.lvl\t!byte\t0<\/font>\n.lvltxt\t!pet\t\"000\"<\/font>,0<\/font><\/code><\/pre>\n\n\n\n
Since the .lvltxt variable is initialized with \u201c000\u201d, we only need to change the digits that are not 0. So if the value in .lvl is 53, we only change the 2nd and 3rd digit and leave the first 0 in place. The .lvltxt will then contain \u201c053\u201d. The function works, roughly this way:<\/p>\n\n\n\n
if value >= 200 then\n\twrite '2' in the first digit\n\tsubtract 200 from the value\nelse if value >= 100 then\n\twrite '1' in the first digit\n\tsubtract 100 from the value\nendif\nfor digit=9 downto 1\n\tnum = digit * 10\n\tif value >= num then\n\t\tconvert digit to petscii char\n\t\twrite petscii-char in 2nd digit\n\t\tsubtract num from value\n\t\tbreak out of for-loop\n\tendif\nendfor\nconvert value to petscii char\nwrite petscii-char in 3rd digit<\/code><\/pre>\n\n\n\n
In the hundred\u2019s we check for either 200 or 100 to see if we need to change the first zero to 2 or 1. For the ten\u2019s a loop is created that counts from 90 down to 10 and checks if we need to change the middle digit. Finally for the one\u2019s, we simply convert the remaining value to a writable char (i.e. add $30 to the value).<\/p>\n\n\n\n
; **************************************************************\n; LVLtoPET\n; **************************************************************\n; Description:\tConvert value in byte to 3-digit decimal string\n; **************************************************************\n; Inputs:\t.lvl = Global variable containing a byte value\n;\t\t.lvltxt = Global variable containing string\n; **************************************************************\nLVLtoPET:\n\t; Local names to zero-page constant to make the code\n\t; a little more readable\n\t.value=$00<\/font>\n\t.digit=$01<\/font>\n\t.num=$02<\/font>\n\n\tlda<\/font>\t.lvl\t; Load Reg A with current level\n\n\t; Check if .lvl is >= 200\n\tcmp<\/font>\t#200<\/font>\n\tbcc<\/font>\t.is100\t; branch to .is100 if .lvl <200\n\tldy<\/font>\t#'2'<\/font>\t; Write '2' to first digit of .lvltxt\n\tsty<\/font>\t.lvltxt\n\tsbc<\/font>\t#200<\/font>\t; Subtract 200 from .lvl\n\tbeq<\/font>\t.allDone; If result = 0, we are done\n\tjmp<\/font>\t.Tens\t\n\t; Check if .lvl is >= 100\n.is100:\n\tcmp<\/font>\t#100<\/font>\n\tbcc<\/font>\t.Tens\t; branch to .Tens if .lvl < 100\n\tldy<\/font>\t#'1'<\/font>\t; Write '1' to first digit of .lvltxt\n\tsty<\/font>\t.lvltxt\n\tsbc<\/font>\t#100<\/font>\t; Subtract 100 from .lvl\n\tbeq<\/font>\t.allDone; If result = 0, we are done\n\t; Check if .lvl contains any tens (10-90)\n.Tens:\n\tldy<\/font>\t#9<\/font>\n\tsty<\/font>\t.digit\t; Store digit in zero-page memory\n\tldy<\/font>\t#90<\/font>\n\tsty<\/font>\t.num\t; Store digit*10 in zero-page memory\n.DoTens\n\tcmp<\/font>\t.num\n\tbcc<\/font>\t.Any10\t; branch to .Any10 if .lvl < .num\n\n\tsta<\/font>\t.value\t; Save current value, we need the a reg\n\tlda<\/font>\t.digit\n\tclc<\/font>\t\t; Clear carry to ensure correct add\n\tadc<\/font>\t#$30<\/font>\t; Add $30 to digit to get petscii char\n\tsta<\/font>\t.lvltxt+1<\/font>;Write digit to 2nd space of .lvltxt\n\n\tlda<\/font>\t.value\t; Restore value into A register\n\tsec<\/font>\t\t; Set carry to ensure correct subtraction\n\tsbc<\/font>\t.num\t; Subtract .num from current .value\n\tjmp<\/font>\t.Ones\n\n.Any10\tcmp<\/font>\t#10<\/font>\n\tbcc<\/font>\t.Ones\t; branch to .Ones if A < 10\n\t; subtract 10 from .value\n\tsta<\/font>\t.value\t; Save current value\n\tlda<\/font>\t.num\t; Subtract 10 from .number\n\tsec<\/font>\n\tsbc<\/font>\t#10<\/font>\n\tsta<\/font>\t.num\n\tlda<\/font>\t.value\t; Restore value into A register\n\tdec<\/font>\t.digit\n\tjmp<\/font>\t.DoTens\n.Ones:\n\tclc<\/font>\t\t; Clear carry to ensure correct add\n\tadc<\/font>\t#$30<\/font>\t; Add $30 to get petscii char\n\tsta<\/font>\t.lvltxt+2<\/font>;Write digit to 3rd space of .lvltxt\n.allDone:\n\trts<\/font><\/code><\/pre>\n\n\n\n
This is, by far, the largest and most complicated helper-function yet. There might be a simpler way of doing it (I hope so) but I have not searched for it.<\/p>\n\n\n\n
Finally we have reached a point where we can actually start to get something written to the screen. Now it is just a question of deciding how the screen should look. At the time I was designing the screen layout for the game, I was unaware that it is possible to write to the bottom right without scrolling the entire screen (See my note on Line to scroll on<\/strong> on the Programming Ramblings<\/a> page)<\/s>. For this reason I am clearing the screen with a black background and creating a frame that sits 1 char in from the border all the way around. This reduces the area for the actual mazes, but there is still plenty of space for some fairly complex mazes.<\/p>\n\n\n\n
The InitSCR function initializes the screen to 40\u00d730 mode. It creates the area that will contain the mazes, level information and a bit of helpful information on how to play the game. The function depends on the following global constants.<\/p>\n\n\n\n
CHROUT=$FFD2<\/font>\t; CHROUT outputs a character (C64 KERNAL API)\nSWAPPER=$FF5F<\/font>\t; Switches between 80x60 and 40x30 modes\nCOLPORT=$0286<\/font>\t; This address contains foreground and\n\t\t; background colors\nWall=113<\/font>\t; PETSCII circle\nWallCol=$0B<\/font>\t; Black\/Darkgray\nSpace=' '<\/font>\t; Char used for spaces<\/code><\/pre>\n\n\n\n
NOTE: SWAPPER is now called SCRMOD and is a new KERNAL function for the CX16 instead of an old function from C128. This means that these examples are no longer working. See the CX16 Programmers reference<\/a> for using SCRMOD<\/p>\n\n\n\n
The following global variables are used by the function.<\/p>\n\n\n\n
.lvl\t\t!byte\t1<\/font>\n.title\t\t!pet\t\"cx16-maze\"<\/font>,0<\/font>\n.helptxt\t!pet\t\"w,a,s,d=move spc=next r=reset q=quit\"<\/font>,0<\/font>\n.lvlstr\t\t!pet\t\"lvl:\"<\/font>,0<\/font>\n.lvltxt\t\t!pet\t\"000\"<\/font>,0<\/font><\/code><\/pre>\n\n\n\n
And finally for the function it self. It makes use of the helper functions that have been explained previously so this function in it self is not very complex.<\/p>\n\n\n\n
; **************************************************************\n; Initializes the screen\n; **************************************************************\n; INPUTS:\tGloabl variables\n;\t\t\t.lvl\n;\t\t\t.title\n;\t\t\t.helptxt\n;\t\t\t.lvlstr\n;\t\t\t.lvltxt\n; **************************************************************\nInitSCR:\n\tlda<\/font>\t$D9<\/font>\t; $D9 contains the number of\n\t\t\t; columns being shown\n\tcmp<\/font>\t#80<\/font>\t; if this is 80, we will switch to 40x30\n\tbeq<\/font>\t.SetIt\t; Set 40 column mode\n\tjmp<\/font>\t.NoSet\n.SetIt:\n\tjsr<\/font>\tSWAPPER\t; Switch screenmode\n.NoSet:\n\tlda<\/font>\t#$01<\/font>\t; Black background, white text\n\tsta<\/font>\tCOLPORT\t; Set Color\n\t\n\tlda<\/font>\t#147<\/font>\t; ClrHome\n\tjsr<\/font>\tCHROUT\t; Clear Screen\n\tlda<\/font>\t#$10<\/font>\t; White background, black text\n\tsta<\/font>\tCOLPORT\t; Set color\n\tldx<\/font>\t#1<\/font>\t; Setup to create top horizontal line\n\tldy<\/font>\t#1<\/font>\n\tjsr<\/font>\tGotoXY\n\tlda<\/font>\t#Space\n\tldx<\/font>\t#38<\/font>\n\tjsr<\/font>\tHLine\t; Draw horizontal line\n\tldx<\/font>\t#28<\/font>\t; Setup to create bottom horizontal line\n\tldy<\/font>\t#1<\/font>\n\tjsr<\/font>\tGotoXY\n\tlda<\/font>\t#Space\n\tldx<\/font>\t#38<\/font>\n\tjsr<\/font>\tHLine\t; Draw horizontal line\n\tldx<\/font>\t#2<\/font>\t; Setup to create left most vertical line\n\tldy<\/font>\t#1<\/font>\n\tjsr<\/font>\tGotoXY\n\tlda<\/font>\t#Space\n\tldx<\/font>\t#26<\/font>\n\tjsr<\/font>\tVLine\t; Draw left most vertical line\n\tldx<\/font>\t#2<\/font>\t; Setup to create right most vertical line\n\tldy<\/font>\t#38<\/font>\n\tjsr<\/font>\tGotoXY\n\tlda<\/font>\t#Space\n\tldx<\/font>\t#26<\/font>\n\tjsr<\/font>\tVLine\t; Draw right most vertical line\n\tlda<\/font>\t#$12<\/font>\t; Set color, white background, red text\n\tsta<\/font>\tCOLPORT\n\tldx<\/font>\t#1<\/font>\t; Set up for title text\n\tldy<\/font>\t#15<\/font>\n\tjsr<\/font>\tGotoXY\n\tldx<\/font>\t#<.title; Write the title text\n\tldy<\/font>\t#>.title\n\tjsr<\/font>\tPrintStr\n\tldx<\/font>\t#1<\/font>\t; Set up for level text (top right corner)\n\tldy<\/font>\t#31<\/font>\n\tjsr<\/font>\tGotoXY\n\tldx<\/font>\t#<.lvlstr; Write the level text\n\tldy<\/font>\t#>.lvlstr\n\tjsr<\/font>\tPrintStr\n\tjsr<\/font>\tLVLtoPET ; Create level as a petscii string\n\tldx<\/font>\t#<.lvltxt\n\tldy<\/font>\t#>.lvltxt\n\tjsr<\/font>\tPrintStr\n\tldy<\/font>\t#2<\/font>\t; Set up for help text (bottom line)\n\tldx<\/font>\t#28<\/font>\n\tjsr<\/font>\tGotoXY\n\tldx<\/font>\t#<.helptxt; Write help text\n\tldy<\/font>\t#>.helptxt\n\tjsr<\/font>\tPrintStr\n\tjsr<\/font>\tFillGA\n\trts<\/font><\/font><\/font><\/code><\/pre>\n\n\n\n
NOTE: First line of above function is not correct. $D9 no longer contains the number of columns, instead it can be found on address $02AE. As mentioned in previous note. SWAPPER no longer exists and is now SCRMOD.<\/p>\n\n\n\n
As you might have noticed, the last line before the rts is actually a call to another helper function that I have not yet described.<\/p>\n\n\n\n
FillGA is a function, who\u2019s only function is to fill the center of the screen, the area within the white border, with \u201cwall\u201d characters. It does this by creating horizontal lines of the \u201cwall\u201d character.<\/p>\n\n\n\n
; **************************************************************\n; Fill the \"gamearea\" with \"wall tiles\"\n; **************************************************************\n; INPUTS:\tGlobal constant WallCol is used.\n; **************************************************************\nFillGA:\n\tlda<\/font>\t#WallCol\t; Set background and foreground\n\tsta<\/font>\tCOLPORT\t\t; color\n\n\tldx<\/font>\t#1<\/font>\t\t; X register holds Y coordinate\n\n.StartOfFill:\n\tinx<\/font>\t\t\t; Increment Y coordinate\n\tstx<\/font>\tTMP0\t\t; Save the Y coordinate in ZP\n\tcpx<\/font>\t#28<\/font>\t\t; If we have reached line 28,\n\tbeq<\/font>\t.EndOfFill\t; We are done, so branch to end\n\tldy<\/font>\t#2<\/font>\t\t; Y register holds X coordinate\n\tjsr<\/font>\tGotoXY\t\t; Place cursor at X, Y coordinates\n\tlda<\/font>\t#Wall\t\t; Load A with 'wall' character\n\tldx<\/font>\t#36<\/font>\t\t; Create a horizontal line that is\n\tjsr<\/font>\tHLine\t\t; 36 characters wide\n\tldx<\/font>\tTMP0\t\t; Restore Y coordinate from ZP\n\tjmp<\/font>\t.StartOfFill\n.EndOfFill\n\trts<\/font><\/code><\/pre>\n\n\n\n
Now with the last helper function in place, the initial screen should look something like this:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Next up: Defining the mazes and getting them drawn on the screen and possibly getting started with some \u201canimation\u201d. Stay tuned for\u00a0part 3<\/a>.<\/p>\n\n\n\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
Continuing from part 1 where I created 3 small functions: GotoXY, HLine and VLine. I am going to create a couple more \u201chelper\u201d functions as well as the function that will initialize…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-43","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=\/wp\/v2\/posts\/43","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=43"}],"version-history":[{"count":3,"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=\/wp\/v2\/posts\/43\/revisions"}],"predecessor-version":[{"id":108,"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=\/wp\/v2\/posts\/43\/revisions\/108"}],"wp:attachment":[{"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=43"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=43"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/techblog.dansbo.dk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=43"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}