BusTimings.s   [plain text]


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;		 DRI: Dave Radcliffe
;
;			More accurately calculate bus timings to minimize drift.
;
;			Borrows from Bob Bradleys version in Mac OS 9 (BPPC:FCodeSrcs:BusTiming.s)
;
;
#include <ppc/asm.h>

#define   kKeyLargoCounterLoOffset	0x15038
#define   kKeyLargoCounterHiOffset	0x1503C

;
; TimeSystemBusKeyLargo(inKeyLargoBaseAddress)
;
; TimeSystemBusKeyLargo - Times how long it takes the PowerPC decrementer to count down
; 1,048,575 ticks.
;
; returns, in r3, the number of KeyLargo timer ticks per 1,048,575 PowerPC decrementer ticks.
;
; trashes r3 - r10
;
; NOTE - interrupts should be disabled when calling this code
;



ENTRY(TimeSystemBusKeyLargo, TAG_NO_FRAME_USED)
			
			lis		r4, 0x000F
			ori		r4, r4, 0xFFFF		; Load decrementer tick count (1,048,575)
			lis		r6, kKeyLargoCounterLoOffset >> 16
			ori		r6, r6, kKeyLargoCounterLoOffset & 0xFFFF ; Counter lo offset
			lis		r7, kKeyLargoCounterHiOffset >> 16
			ori		r7, r7, kKeyLargoCounterHiOffset & 0xFFFF ; Counter hi offset
			lwbrx	r8, r6, r3			; Read low 32-bits of counter
			lwbrx	r9, r7, r3			; Read hi 32-bits of counter
			
			; Set up decrementer and wait for it to tick down
			
			mtdec	r4					; Set decrementer to 1,048,575
			isync
			
NewDecrementerLoop:
			mfdec	r5					; Read current decrementer value
			cmpwi	r5, 0				; Check if decrementer is zero
			bgt+	NewDecrementerLoop		; If not yet to zero, keep looping
			sync
			
			; Read current value of KeyLargo to get delta time
			
			lwbrx	r4, r6, r3			; Load low 32-bits of timer (latches all 64 bits)
			lwbrx	r5, r7, r3			; Load high 32-bits of timer (clear latch)
			
			; Calculate difference
			subf	r3, r8, r4			; Subtract low bits (ignore wrap)
			blr							; Return