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--- sd:getting_stuff_done_in_assembly_language [2026/06/01 02:34] – appledog
+++ sd:getting_stuff_done_in_assembly_language [2026/06/01 14:44] (current) – appledog
@@ Line 123: / Line 123: @@
 </codify>
-=== Moving right along
+=== Random Numbers
-We're not going to provide examples of other basic stuff too much, but we will show you this demo program that might help you.
+Random numbers are really useful. Here, we demonstrate the xorshift generator that comes standard in the INT $13 math services library.
 <codify armasm>
+; Example 4: Random Numbers
 .address $C000
@@ Line 170: / Line 172: @@
     ; ============================================================================
-    ; AH=00h - Random Number (Galois LFSR)
+    ; AH=00h - Random Number
     ; Input:  None
-    ; Output: B = random 16-bit number (1-65535)
+    ; Output: A, B = random 16-bit numbers (1-65535)
-    ; Notes:  Uses polynomial x^16 + x^14 + x^13 + x^11 + 1 ($B400)
+    ; Notes:  Uses xorshift
-    ;         Single seed, clean maximal-length sequence
     ; ============================================================================
@@ Line 201: / Line 202: @@
     ; ============================================================================
-== Terminal Cursor Operation
+=== Putting It All Together: Menus
+The engine for all these games we love such as Bard's Tale, Ulima, Nethack, King's quest, Alice in wonderland, Maniac Mansion, all were based on text engines. If the engine is good, a modicum of graphics will suffice. The big deal with retro games is that it's all about the gameplay, and the graphics serve the gameplay. What people miss in today's games is that the graphics attempt to look so good that they can replace gameplay. But it goes without saying, that even so, King's Quest was a "better game" than Tomb Raider, even though Tomb Raider is such an amazing game in and of itself. It fell into that trap, replacing gameplay with graphics. I mean, Tomb Raider pulled it off because it was so awesome, but King's Quest, OMG, hard to say which one is better really.
+The first thing you need to do is write some menus. Fundamentally printing the menus is easy, but you may wish to color the title of the menus, so you will need to learn how to do colored text. Also, you want to do input validation, so let's work on that too.
+First, colored text. it's exactly the same as writing normal text, except you write a color byte. Here, i've added a special function to change the default color text is written in TTY mode. You can also set the color of text at an X and Y location, but that can become tedious because you have to do it character by character.
+<codify armasm>
+; Example 5: Menus and Colored Text
+.address $C000
+start:
+    ; Clear screen.
+    LDAH $10
+    INT $10
+    ; Set cursor position to X=0, Y=10.
+    LDX #0
+    LDY #10
+    LDAH $15    ; set cursor position
+    INT $10
+    LDAH $50    ; Set teletype text default color
+    LDAL $2F    ; color data in AL: BG=2, FG=F (here, orange)
+    INT $10
+    LDBLX @str_menu_title
+    LDAH $66    ; BASIC IO_PUTSTRING
+    INT 0x05
+    LDAH $50    ; Set teletype text default color
+    LDAL $29    ; color data default for mode 1
+    INT $10
+    LDBLX @str_menu
+    LDAH $66    ; BASIC IO_PUTSTRING
+    INT 0x05
+    LDAH $02    ; blocking getkey
+    INT 0x10
+    RET
+str_menu_title:
+   .bytes "  Choose your class:", 13, 10, 13, 10, 0
+str_menu:
+   .bytes "    1) Human", 13, 10
+   .bytes "    2) Elf", 13, 10
+   .bytes "    3) Half-elf", 13, 10
+   .bytes "    4) Dwarf", 13, 10
+   .bytes "    5) Orc", 13, 10, 13, 10
+   .bytes "  Your choice [1-5, q]? ", 0
+</codify>
+== Major Concept: The Cursor
 Before we proceed, it's nice to be able to completely control the terminal. That means, instead of relying solely on PRINT and INPUT we have character level control.
@@ Line 215: / Line 273: @@
 <codify armasm>
+; Example 6: The Cursor
 .address $C000
@@ Line 262: / Line 322: @@
 </codify>
-== Writing the game engine
+=== Keeping Track Yourself
-The engine for all these games we love such as Bard's Tale, Ulima, Nethack, King's quest, Alice in wonderland, Maniac Mansion, all were based on text engines. If the engine is good, a modicum of graphics will suffice. The big deal with retro games is that it's all about the gameplay, and the graphics serve the gameplay. What people miss in today's games is that the graphics attempt to look so good that they can replace gameplay. But it goes without saying, that even so, King's Quest was a "better game" than Tomb Raider, even though Tomb Raider is such an amazing game in and of itself. It fell into that trap, replacing gameplay with graphics. I mean, Tomb Raider pulled it off because it was so awesome, but King's Quest, OMG, hard to say which one is better really.
+In a game, you don't really need the cursor. So instead of using a teletyle print function and calling INT $10, AH=$16, you can just give the values you want for your game screen. Here's an example of putting a character anywhere you want and using any color. Although it looks a bit trivial this ends up proving to be a very useful pattern.
-=== Menus
+<codify armasm>
-The first thing you need to do is write some menus. Fundamentally printing the menus is easy, but you may wish to color the title of the menus, so you will need to learn how to do colored text. Also, you want to do input validation, so let's work on that too.
+; Example 7: put_ccxy
-First, colored text. it's exactly the same as writing normal text, except you write a color byte. Here, i've added a special function to change the default color text is written in TTY mode. You can also set the color of text at an X and Y location, but that can become tedious because you have to do it character by character.
-<codify armasm>
 .address $C000
 start:
-    ; Clear screen.
-    LDAH $10
-    INT $10
-    ; Set cursor position to X=0, Y=10.
     LDX #0
     LDY #10
-    LDAH $15    ; set cursor position
+    LDC $03
+    LDB #'A'
+    CALL @put_ccxy
+    RET
+; put_ccxy(B, X, Y, C)
+; Put Character and Color at X, T
+; INPUT:
+;     B -- ascii code
+;     X -- x location on screen
+;     Y -- y location on screen
+;     C -- hi nibble (BG) and low niblle (FG)
+;
+put_ccxy:
+    ; Write character in AL at X, Y
+    LDAH $11
     INT $10
-    LDAH $50
+    ; Set color at X, Y
-    LDAL $2F    ; color data in AL: BG=2, FG=F (here, orange)
+    ; Input:  CL = color data already sey
+    LDAH $13
     INT $10
-    LDBLX @str_menu_title
+    RET
-    LDAH $66    ; BASIC IO_PUTSTRING
+</codify>
-    INT 0x05
-    LDAH $50
+== DEFCHAR Graphics
-    LDAL $29    ; color data default for mode 1
+Mode 1 Graphics is the first step in our graphics adventure. Mode 1 Graphics has two limitations. One, since you are redefining the character set to do graphics, everything must be drawn using 8x8 monocolor stamps (like characters). Two, you can only have 256 of them.
+First, use int10h_font_set_char and int10h_glyph_pack to define the character:
+    ; ----------------------------------------------------------------------------
+    ; AH=49h - glyph_pack -- pack an 8x8 grid and install it as a font glyph
+    ;   Input:    AL  = character code (0..255)
+    ;             ELM = pointer to 64 source chars (8 rows x 8 cols, row-major)
+    ;   Encoding: ' ' (space) or 0 -> bit clear; any other byte -> bit set
+    ;             MSB = leftmost column
+    ;   Writes:   8 bytes at petscii_data + charcode*8
+    ; ----------------------------------------------------------------------------
+You can also use AH=48h if you have the pre-compiled bytes:
+    ; ----------------------------------------------------------------------------
+    ; AH=48h - font_set_char -- install an 8-byte glyph into the font
+    ;   Input:    AL  = character code (0..255)
+    ;             ELM = pointer to 8 source bytes
+    ;   Writes:   8 bytes at petscii_data + charcode*8
+    ;   Destroys: A   (BLX/FLD/C/K preserved)
+    ; ----------------------------------------------------------------------------
+Here's how it works:
+<codify armasm>
+; Example 8: DEFCHAR
+.address $C000
+start:
+    LDELM @smiley
+    LDAH $49
+    LDAL #'!'  ; replace the ! character
     INT $10
+    RET
+    ; Now the ! is replaced by a smiley.
+smiley:
+    .bytes "        "
+    .bytes "  #  #  "
+    .bytes "        "
+    .bytes " #    # "
+    .bytes " #    # "
+    .bytes "  ####  "
+    .bytes "        "
+    .bytes "        "
+</codify>
-    LDBLX @str_menu
+Unfortunately, the conversion to a smiley is permanent! At least, until you reboot the system (or change it back).
-    LDAH $66    ; BASIC IO_PUTSTRING
-    INT 0x05
+=== Using this concept in Mode 3
+Now, interestingly enough, you can translate this graphics technique directly into Mode 3. First, let's adapt BASIC's DRAWCHAR (INT $18 AH=$60 and $61) so we can blit characters in Mode 3.
+    ; ----------------------------------------------------------------------------
+    ; AH=61h -- stamp_char -- render a glyph onto mode-3, transparent background
+    ;   Input:  AL = character code (0..255)
+    ;           X  = base X (pixel)
+    ;           Y  = base Y (pixel)
+    ;           CL = foreground color (only looks at low nibble)
+    ;   Uses:   INT 18h AH=01h (plot pixel);
+    ;           glyph source = VM1_CHAR_ROM ($01E000)
+    ; ----------------------------------------------------------------------------
+We'll toss in some fancy arrow key movement and a q for quit key, to spice things up:
+<codify armasm>
+; Example 9: stamp_char
+.address $C000
+start:
+    ; Switch to mode 3
+    LDA $4003   ; 40h = set mode, 3 = mode 3
+    INT $10
+    ; Clear screen
+    LDAH $20
+    INT $10
+    LDX #0
+    LDY #0
+loop:
+    ; draw an 'x' at X, Y
+    LDAL #'x'
+    LDAH $61    ; stamp char
+    LDC #9      ; dark blue
+    INT $18
     LDAH $02    ; blocking getkey
     INT 0x10
-    RET
+    LDAH $60    ; erase char
+    LDC #0      ; black background
+    INT $18
+    CMP AL, #'q'
+    JZ @exit
+    CMP AL, #'Q'
+    JZ @exit
-str_menu_title:
+    CMP AL, #128
-   .bytes "  Choose your class:", 13, 10, 13, 10, 0
+    JZ @move_left
+    CMP AL, #129
+    JZ @move_down
+    CMP AL, #130
+    JZ @move_up
+    CMP AL, #131
+    JZ @move_right
-str_menu:
+    JMP @loop
-   .bytes "    1) Human", 13, 10
-   .bytes "    2) Elf", 13, 10
+move_left:
-   .bytes "    3) Half-elf", 13, 10
+    DEC X
-   .bytes "    4) Dwarf", 13, 10
+    DEC X
-   .bytes "    5) Orc", 13, 10, 13, 10
+    JNN @loop   ; if X didn't go negative, just continue.
-   .bytes "  Your choice [1-5, q]? ", 0
+    LDX #0      ; Don't allow X to go below 0.
+    JMP @loop
+move_right:
+    INC X
+    INC X
+    CMP X, #312
+    JNC @loop
+    ; C is set if X was >= 312
+    LDX #311
+    JMP @loop
+move_up:
+    DEC Y
+    DEC Y
+    JNN @loop   ; if Y didn't go negative, just continue.
+    LDY #0
+    JMP @loop
+move_down:
+    INC Y
+    INC Y
+    CMP Y, #191
+    JNC @loop
+    LDY #191
+    JMP @loop
+exit:
+    ; Switch back to mode 1
+    LDA $4001
+    INT $10
+    RET
 </codify>
-== DEFCHAR Graphics
+=== Multi-Color Character Graphics in Mode 3
-Mode 1 Graphics is the first step in our graphics adventure. Mode 1 Graphics has two limitations. One, since you are redefining the character set to do graphics, everything must be drawn using 8x8 monocolor stamps (like characters). Two, you can only have 256 of them.
+The way to do multicolor graphics is to use different tiles for each color component. For example, you can have one tile with the blue pixels and the other tile with the yellow pixels. When you draw the tiles on top of each other, it will produce a multicolor effect.
-First, use int10h_font_set_char and int10h_glyph_pack to define the character:
+== Music
+Randomly, in the middle of nowhere, we will explain to you; yes, you can play music in your game. Our PLAY statement is multi-voice; please see the relevant documentation for more information. For now, here's a short little ditty reminiscent of England in the summer of the 1600's.
-    letter_a:
+<codify BASIC>
-        .bytes "   ██   "
+PLAY "XV1 MN W5 XP3000 O4 L4 T180"
-        .bytes "  █  █  "
+PLAY "XV2 MN W5 XP3500 O3 L4 T180"
-        .bytes " █    █ "
+PLAY "XV1 F1"
-        .bytes " █    █ "
+PLAY "XV2 R1"
-        .bytes " ██████ "
+PLAY "XV1 A4. B-8 > C4 C4 <"
-        .bytes " █    █ "
+PLAY "XV2 R1"
-        .bytes " █    █ "
+PLAY "XV1 B-4 A4 G4 F4"
-        .bytes "        "
+PLAY "XV2 R1"
+PLAY "XV1 > C2 C4. C8 <"
+PLAY "XV2 R2 F4. F8"
+PLAY "XV1 > C4 D2 C4 <"
+PLAY "XV2 F4 < B-2 A4"
+PLAY "XV1 B-4 A4 G4 G4"
+PLAY "XV2 B-4 F4 > C4 C4"
+PLAY "XV1 A1"
+PLAY "XV2 < F1"
+</codify>
+Now you may wonder, why incorporate a BASIC example? Here, we can use BASIC to quickly test musical ideas. When it's time to put them into a game, we do this:
 <codify armasm>
-    LDBLX @letter_a
+; Example 10: Music
-    LDFLD @buf
-    LDAH $49        ; int10h_glyph_pack
-    INT 0x10
+.address $C000
-buf:
+    LDELM @fair_phyllis
-    .bytes 0,0,0,0,0,0,0,0
+    LDAH $52        ; Play Song
+    INT $11
+    RET
+fair_phyllis:
+    .bytes "XV1 MN W5 XP3000 O4 L4 T180"
+    .bytes "XV2 MN W5 XP3500 O3 L4 T180"
+    .bytes "XV1 F1 "
+    .bytes "XV2 R1 "
+    .bytes "XV1 A4. B-8 > C4 C4 < "
+    .bytes "XV2 R1 "
+    .bytes "XV1 B-4 A4 G4 F4 "
+    .bytes "XV2 R1 "
+    .bytes "XV1 > C2 C4. C8 < "
+    .bytes "XV2 R2 F4. F8 "
+    .bytes "XV1 > C4 D2 C4 < "
+    .bytes "XV2 F4 < B-2 A4 "
+    .bytes "XV1 B-4 A4 G4 G4 "
+    .bytes "XV2 B-4 F4 > C4 C4 "
+    .bytes "XV1 A1 "
+    .bytes "XV2 < F1", 0
+</codify>