initial commit

clear screen and basic key handling, no life related code just yet

3 files changed, 125 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..5baabe9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.o
+*.prg
diff --git a/Makefile b/Makefile
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index 0000000..28e66d4
--- /dev/null
+++ b/Makefile
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+run: life.prg
+	x64 life.prg
+life.prg: life.asm
+	cl65 -C /usr/share/cc65/cfg/c64-asm.cfg -u __EXEHDR__ $^ -o $@
diff --git a/life.asm b/life.asm
new file mode 100644
index 0000000..d04fe7f
--- /dev/null
+++ b/life.asm
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+.export init
+
+numL := $02 ; low byte of a 16-bit number
+numH := $03 ; low byte of a 16-bit number
+key_buf_len := $c6
+key_buf := $0277
+
+; initialisation: clear screen and set num to point to screen mem start
+init:
+  jsr clear_screen
+  ldy #$00
+  lda #$04
+  sta numH
+  lda #$00
+  sta numL
+  jmp loop
+
+; main loop
+loop:
+  ; clear keyboard buffer; otherwise program will always think a key is pressed
+  lda #$00
+  sta key_buf_len
+
+  jsr key_wait
+  lda key_buf
+  jsr print_byte
+
+  ;pha
+  ;lda #1
+  ;jsr a16b
+  ;pla
+
+  inc numL
+  beq inc_H
+  jmp dinc_H
+  inc_H:
+    inc numH
+dinc_H:
+  cmp #$30
+  bne loop
+  jmp end
+
+; loops until keyboard buffer has any number of characters in it
+key_wait:
+  ; init: pha to retrieve before rts, to keep A intact
+  pha
+key_wait_loop:
+  lda key_buf_len ; shows how many keys are in key buffer
+  cmp #$00
+  beq key_wait_loop
+  pla
+  rts
+
+; clear the screen by filling it with spaces
+; destroys: A, Y, numL & numH (in memory)
+clear_screen:
+  ; init: y = 0 because you can't use indirect addressing without a register,
+  ; load start of screen memory into numL/numH
+  ldy #0
+  lda #$04
+  sta numH
+  lda #$00
+  sta numL
+  jmp cs_loop
+
+  ; add 1 to high bit, return if $08 (screen mem only goes to $07ff)
+  add_carry:
+    inc numH
+    lda numH
+    cmp #$08
+    bne cs_loop
+    rts
+
+  ; store a space in current screen mem address as specified by num, branch if
+  ; number overflows to 0 (i.e. add 1 to high bit of 16 bit number)
+  cs_loop:
+    lda #$20
+    sta (numL),y
+    inc numL
+    beq add_carry
+    jmp cs_loop
+
+; print byte in A at location in num
+print_byte:
+  ; init: store original A in X and push Y to stack
+  tax
+  tya
+  pha
+  txa
+
+  ldy #$00
+  sbc #$40 ; convert (very vaguely) to c64 screen codes
+  sta (numL),y
+
+  ; deinit: pull Y from stack and transfer original A from X to A
+  pla
+  tay
+  txa
+  rts
+
+; adds A to num (16 bit) (NOT WORKING)
+; destroys: X
+a16b:
+  pha
+  tsx
+  lda numL
+  adc $0100,x
+  sta numL
+  bcs a16b_carry
+  jmp a16b_end
+  a16b_carry:
+    inc numH
+  a16b_end:
+    clc
+    pla
+    rts
+
+end:
+  brk