Rabbit 2000, User Manual

Page: 40 / 230

34 Rabbit 2000 Microprocessor
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 3-7 on page 34. Once the mapping
has been performed by inverting bit 15 on both numbers, the comparisions can be done as
if the numbers 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 3-7. 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...