| ← B991 | Map | B9C2 → |
B9B4: Emit one square-wave cycle (the sound primitive)
|
The one place any sound effect reaches the speaker. The SOUND FX option therefore mutes them all together.
|
|||||||||||
|
|||||||||||
|
With SOUND FX off, the options-menu redraw patches this routine's first byte so it returns at once. With SOUND FX on, it restores that byte.
Seven effects share this counted-step shape, written here once: sfx_death_burst, sfx_spawn, sfx_chew, sfx_bomb_detonate, sfx_bat_grows, sfx_bat_shrinks and sfx_net_flash. Each is polled once a frame and returns at once while its counter reads 0. A raise of that counter arms it, and one step past the last clears it.
Every armed call bumps the counter, derives that step's pitch period and emits a short burst here. Five of the seven take the period from a table indexed by the counter, bumped before the index is taken. Those tables therefore never have their first two bytes read.
|
|||||||||||
| sound_blip | B9B4 | LD A,$10 | Speaker high level | ||||||||
|
The mute is this routine's first byte. The SOUND FX handler also has a path that would disable the two speaker writes, but that branch never runs.
|
|||||||||||
| sound_lvl_hi | B9B6 | OUT ($FE),A | Drive the speaker high. | ||||||||
|
Each half-cycle holds the speaker in one state for as long as the pitch period sets. A period of 0 counts as 256.
|
|||||||||||
| B9B8 | LD B,D | Load the delay count from the pitch period in D. | |||||||||
| sound_hi_delay | B9B9 | DJNZ sound_hi_delay | Hold the high level for D loop iterations. | ||||||||
| B9BB | XOR A | Speaker low level | |||||||||
| sound_lvl_lo | B9BC | OUT ($FE),A | Drive the speaker low. | ||||||||
| B9BE | LD B,D | Reload the delay count from D. | |||||||||
| sound_lo_delay | B9BF | DJNZ sound_lo_delay | Hold the low level for D iterations, then return. | ||||||||
| B9C1 | RET | ||||||||||
| ← B991 | Map | B9C2 → |