Fix occasional bug in delay() and usleep() (#333)

Author: runer112

src/ce/delay.src

@@ -24,13 +24,14 @@ _delay:
 ; ^ 13 cc
 ; Get the start time ASAP.
 	push	hl
-	call	_clock			; a = 0, euhl = start
+	call	_clock			; euhl = start
 ; ^ 32 + cc(_clock) cc
 ; 22 + 48 + 13 + 32 + 48 + 75 = 238 cc until start time captured
 	ex	(sp), hl		; hl = millis
 	push	de
 ; Convert millis to clock ticks.
 	dec.s	hl
+	xor	a, a			; a = 0
 	ld	e, a			; euhl = millis - 1
 .multiplier_fp := (256 * 32768 + 1000 - 1) / 1000
 assert .multiplier_fp shr 24 = 0

src/ce/sleep.src

@@ -25,7 +25,7 @@ _sleep:
 ; ^ 13 cc
 ; Get the start time ASAP.
 	push	hl
-	call	_clock			; a = 0, euhl = start
+	call	_clock			; euhl = start
 ; ^ 32 + cc(_clock) cc
 ; 22 + 48 + 13 + 32 + 48 + 75 = 238 cc until start time captured
 	ex	(sp), hl		; hl = seconds

src/ce/ticksleep.src

@@ -31,7 +31,7 @@ _ticksleep:
 ; Get the start time ASAP.
 	push	de
 	push	hl
-	call	_clock			; a = 0, euhl = start
+	call	_clock			; euhl = start
 ; ^ 42 + cc(_clock) cc
 ; 22 + 103 + 42 + 48 + 75 = 290 cc until start time captured
 ; Join common code for the rest.

src/ce/usleep.src

@@ -26,13 +26,14 @@ _usleep:
 ; ^ 21 cc
 ; Get the start time ASAP.
 	push	hl
-	call	_clock			; a = 0, euhl = start
+	call	_clock			; euhl = start
 ; ^ 32 + cc(_clock) cc
 ; 22 + 48 + 21 + 32 + 48 + 75 = 246 cc until start time captured
 	ex	(sp), hl		; hl = usec
 	push	de
 ; Convert usec to clock ticks.
 ; https://docs.google.com/spreadsheets/d/1mZwDn6rEw3-uSQPuy48Ez0UWIvnggLiJxWft4u4OtcU/edit?usp=sharing
+	xor	a, a			; a = 0
 	ld	e, a			; euhl = usec
 .multiplier := 114
 	ld	bc, .multiplier		; aubc = multiplier

src/std/shared/clock.src

@@ -6,7 +6,7 @@ mpTmr2Counter := 0F20010h
 	public	_clock
 _clock:
 assert mpTmr2Counter shr 8 and $FF = 0
-; O: a = 0, euhl = ticks
+; O: euhl = ticks
 assert mpTmr2Counter and $A7 = 0
 ; FO: s = 0, z = 0, h = 0, p/v = 0, n = 0, c = 0
 ; CC: 79 + cc(_atomic_load_increasing_32)