In my electronics projects I frequently use LEDs when testing outputs and in debugging code, so I thought I would add some to BMDS. The main control form now sports an optional set of 8 LEDs, which are mapped to a configurable address. Whenever the address is written to the LEDs light according to the bit pattern of the value written; 1 = on, 0 = off.
Initially I stuck with a simple blink program but then decided to get ambitious and having enjoyed Michael Knight’s KITT car many years ago, decided to try cycling the LEDs back and forth. Could also be a Cylon, as I like Battlestar Galactica too.
It is a long time since I wrote 6502 code so was pleasantly surprised when it worked first time. I only had to tweak the delay a few times to have it look as I wanted. I guess the decades of programming since my first attempts in the late 70s have taught me something!
The code is fairly simple, using a table of bit masks to select the relevant bit for each LED. I put the LEDs address at $B000 with the other I/O addresses.
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// Assembler for processor 6502
//
// File "knight_rider_led.asm"
// 04 Jun 2020, 09:29
//
0001 //
0002 // Testing LEDs with a Knight Rider like sequence
0003 //
0004 // Assemble with 'Write code to memory' set
0005 // Then in TANBUG use G400 to run
0006 // Stop with Reset button
0007 //
0008
0009 0100 leds equ $B000
0010
0011 0102 org $400,CODE // Start of program code
0012
0013 0102 A2 00 start ldx #0
0014 0104 A9 00 lda #0
0015 0106 1D 60 01 loopLeft ora ledOn,X // LED x on
0016 0109 8D 00 01 sta leds
0017 010C 20 4F 01 jsr delay
0018 010F 1D 61 01 ora ledOn+1,x // then LED x+1 on
0019 0112 8D 00 01 sta leds
0020 0115 20 4F 01 jsr delay
0021 0118 3D 68 01 and ledOff,X // LED x off, leaving x+1 on
0022 011B 8D 00 01 sta leds
0023 011E 20 4F 01 jsr delay
0024 0121 20 4F 01 jsr delay
0025 0124 E8 inx
0026 0125 E0 07 cpx #7 // Cycle through all LEDs
0027 0127 D0 DD bne loopLeft
0028
0029 0129 A9 00 lda #0
0030 012B 1D 60 01 loopRight ora ledOn,X // Then do in reverse
0031 012E 8D 00 01 sta leds
0032 0131 20 4F 01 jsr delay
0033 0134 1D 5F 01 ora ledOn-1,X
0034 0137 8D 00 01 sta leds
0035 013A 20 4F 01 jsr delay
0036 013D 3D 68 01 and ledOff,X
0037 0140 8D 00 01 sta leds
0038 0143 20 4F 01 jsr delay
0039 0146 20 4F 01 jsr delay
0040 0149 CA dex
0041 014A D0 DF bne loopRight
0042 014C 4C 02 01 jmp start // And around again
0043
0044 //
0045 // Delay, set by trial
0046 //
0047 014F 48 delay pha // Save state
0048 0150 8A txa
0049 0151 48 pha
0050 0152 A2 14 ldx #20 // Outer loop
0051 0154 A0 00 loop1 ldy #0
0052 0156 88 loop2 dey // Inner loop 256 times
0053 0157 D0 FD bne loop2
0054 0159 CA dex
0055 015A D0 F8 bne loop1
0056 015C 68 pla // Restore state
0057 015D AA tax
0058 015E 68 pla
0059 015F 60 rts
0060
0061 // Table of bits for switching each LED on or off
0062
0063 0160 01 ledOn fcb %0000.0001 // On
0064 0161 02 fcb %0000.0010
0065 0162 04 fcb %0000.0100
0066 0163 08 fcb %0000.1000
0067 0164 10 fcb %0001.0000
0068 0165 20 fcb %0010.0000
0069 0166 40 fcb %0100.0000
0070 0167 80 fcb %1000.0000
0071
0072 0168 FE ledOff fcb %1111.1110 // Off
0073 0169 FD fcb %1111.1101
0074 016A FB fcb %1111.1011
0075 016B F7 fcb %1111.0111
0076 016C EF fcb %1110.1111
0077 016D DF fcb %1101.1111
0078 016E BF fcb %1011.1111
0079 016F 7F fcb %0111.1111
0080
0081 end
Assembly generated 0 errors and 0 warnings
The ASM file is available in the examples/Microtan 65 folder on GitHub along with the rest of the BMDS source code.