Fast RNG algorithm

[zoranc]

Just found a nice pseudo random number generator algorithm suitable for S1C88. It uses only 16 bit numbers and is very fast when implemented in assembly:

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uint16_t xs = 1; // initialised with a non-zero seed
xs ^= xs << 7
xs ^= xs >> 9
xs ^= xs << 8

Here is implementation for several processors including z80 which is very similar to S1C88: https://github.com/impomatic/xorshift798

[YasaSheep]

Nice! Here's a 36 cycle version in Epson syntax, best I could get:

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xorshift:
	LD HL,#XSHFT
	LD BA,[HL]
	RR B
	RR A
	XOR [HL],A
	LD BA,[HL]
	RR B
	XOR A,B
	XOR [HL],A
	INC HL
	LD [HL],A
	RET

xor is fairly limited (the only two register version is a,b) so I switched to xoring in memory where possible.