Reorganized folders, added pal-adapter io device folder

pat80-computer/software/z80-assembly/os/drivers/sn76489.asm

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+; TI SN76489 sound chip display driver
+; @author Daniele Verducci
+;
+; USAGE:
+; call Snd_init                     <-- inits sound (and silences default tone)
+; call Snd_beep                     <-- system beep
+
+; Sound card is on port 1
+SND_DATA_REG: EQU IO_1
+
+; Init device (silence all channels)
+; Bits meaning:
+; 1 R0 R1 R2 A0 A1 A2 A3
+;   Bit0 is 1
+;   Bit1,2,3 select the channel: 001, 011, 101, 111(noise)
+;   Bit4,5,6,7 selecy the attenuation (0000=full volume, 1111=silent)
+Snd_init:
+    ; silence ch1
+    ld a,%10011111
+    out (SND_DATA_REG),a
+    ; silence ch2
+    ld a,%10111111
+    out (SND_DATA_REG),a
+    ; silence ch3
+    ld a,%11011111
+    out (SND_DATA_REG),a
+    ; silence noise ch
+    ld a,%11111111
+    out (SND_DATA_REG),a
+    ret
+
+; Plays the system beep.
+Snd_beep:
+    ; ch1 max volume
+    ld a,%10010000
+    out (SND_DATA_REG),a
+    ; play beep freq
+    ld a,%10000000
+    out (SND_DATA_REG),a
+    ld a,%00001000
+    out (SND_DATA_REG),a
+    ; wait
+    ld bc, (TIME_DUR_MILLIS * 100)
+    call Time_delay55
+    ; silence ch1
+    ld a,%10011111
+    out (SND_DATA_REG),a
+    ret
+
+; Sets the attenuation value for a channel
+; @param a Channel (0, 1, 2, 3(Noise))
+; @param c Attenuation (0 to 16)
+; Snd_setAtt:
+;     cp a, 0
+