Jameco-electronics Rabbit 3000 Manual de usuario descargar pdf (Pagina 52)

User’s Manual 43

A>B (!S & !V & !Z) v (S & V)

A<B (S & !V) v (!S & V & !Z)

A==B

A>=B

A<=B

Another method of doing signed compare is to first map the signed integers onto unsigned

integers by inverting bit 15. This is shown in Figure 3-8. Once the mapping has been per-

formed by inverting bit 15 on both numbers, the comparisions can be done as if the num-

bers were unsigned integers. This avoids having to construct a jump tree to test the

overflow and sign flags. An example is shown below.

; test HL>5 for signed integers

LD DE,65535-(5+0x08000) ; 5 mapped to unsigned integers

LD BC,0x08000

ADD HL,BC ; invert high bit

ADD HL,DE ; 16 clocks to here

; carry now set if HL>5 - opportunity to jump on carry

SUBC HL,HL ; HL-HL-C ; if C on result is -1, else zero

BOOL HL ; 22 clocks total - true if HL>5 else false

Figure 3-8. Mapping Signed Integers to Unsigned Integers by Inverting Bit 15

3.4.5 Atomic Moves from Memory to I/O Space

To avoid disabling interrupts while copying a shadow register to its target register, it is

desirable to have an atomic move from memory to I/O space. This can be done using LDD

or LDI instructions.

LD HL,sh_PDDDR ; point to shadow register

LD DE,PDDDR ; set DE to point to I/O reg

SET 5,(HL) ; set bit 5 of shadow register

; use ldd instruction for atomic transfer

IOI ldd ; (io DE)<-(HL) HL--, DE--

When the LDD instruction is prefixed with an I/O prefix, the destination becomes the I/O

address specified by DE. The decrementing of HL and DE is a side effect. If the repeating

instructions LDIR and LDDR are used, interrupts can take place between successive itera-

tions. Word stores to I/O space can be used to set two I/O registers at adjacent addresses

with a single noninterruptable instruction.

0111...

000...

111...

100...

1111...

100...

011...

000...

1 2 ... 47 48 49 50 51 52 53 54 55 56 57 ... 348 349

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Jameco-electronics Rabbit 3000 Manual de usuario Pagina 52