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--- sd:part_ii_writing_games_in_assembly_language [2026/04/02 19:59] – appledog
+++ sd:part_ii_writing_games_in_assembly_language [2026/04/03 05:30] (current) – appledog
@@ Line 3: / Line 3: @@
 == Introduction
-At the end of [[SD-8516 Stellar BASIC]] we wrote a game called [[sdb:robots|ROBOTS.BAS]]. This was a fun little take on the classic CHASE or 'robots' game from the bsd games collection. Now, for our study of Assembly Language, let's write a similar game: 'robots.asm'.
+At the end of [[SD-8516 Stellar BASIC]] we wrote a game called [[sdb:robots|ROBOTS.BAS]]. This was a fun little take on the classic CHASE or 'robots' game from the bsd games collection. Now, for our study of Assembly Language, let's write a similar game: 'robots.asm'. Don't worry if you're not familiar with the BASIC version of this program; everything will be explained here.
 == Part 1: Assembling and Running a Program
-An assembly language program should start with an .address the bytes will be compiled from. Our program therefore begins:
+Just like BASIC programs have line numbers, or C programs have #include statements, an assembly language program should start with an ''.address'' statement. This will tell the assembler where to start assembling the program. This enables us to use labels like ''start:'' and ''message:''. The assembler can calculate the exact address of the label in relative to the ''.address'' when assembling the program.
 <codify armasm>
@@ Line 280: / Line 280: @@
 The second compare has a protective function.
-    CMP AL, #123
+    CMP AL, #123             ; if AL >= 123, set carry.
     ; If it's '{' or anything after, skip:
     JC @gi_check_keys        ; Jump if Carry = Jump if AL > 'z' (al >= '}')
@@ Line 289: / Line 289: @@
 Visual summary:
+^ AL Value ^ Meaning ^ What Happens ^
-| AL Value | Meaning | What Happens |
 | < 97 | before ''a'' | Skipped. |
-| 97-122 | ''a'' to ''z'' | SUB #32 becomes 'A' to 'Z' |
+| 97-122 | ''a'' to ''z'' | SUB #32 becomes 'A' to 'Z'. |
-| >= 123 | after 'z'\\ //('}' and up)// |  Skipped |
+| >= 123 | after 'z'\\ //'}' and up// | Skipped. |
+=== Processing player input
+We will use the HJKL convention:
+<codify armasm>
+gi_check_keys:
+    CMP AL, #'H'
+    JZ @move_left
+    CMP AL, #'J'
+    JZ @move_down
+    CMP AL, #'K'
+    JZ @move_up
+    CMP AL, #'L'
+    JZ @move_right
+    CMP AL, #'Q'
+    JZ @quit_game
+    RET
+</codify>
+This is easy enough; after a CMP, if the values are equal then the zero flag is set. We will use JZ (jump if zero flag is set) to go to the correct routine.
+=== processing each command
+<codify armasm>
+move_left:
+    LDAL [@PX]        ; load variable PX into register AL.
+    DEC AL            ; X = X - 1 (move to the left!)
+    CMP AL, #1        ; bounds check. Is the player on the border?
+    JC @ml_ok         ; AL >= 1, so it's ok. skip to ml_ok,
+    LDAL #1           ; If  we reach here the player was on the border; set position to 1.
+ml_ok:
+    STAL [@PX]        ; pass! Save the player's location.
+    RET
+</codify>
+This is the move_left command. We begin by loading the player's X location into AL and executing the move. After a bounds check, we save the variable. All of the other moves operate this way.