ZiLOG Z80 Handbook page 170

digits. As there will be eight ASCII characters representing 0
(ASCII 30H) or 1 (ASCII 31), a buffer of eight bytes is allocated
to hold the results of the conversion. The bits in the operand to be
converted are tested one at a time by shifting the A register right in
a rotate shift. The rotate could be either an RRCA rotate or an RRC
A rotate. Both perform the same action (the RRCA is compatible
with the 8080 rotate of this kind). Since the RRCA takes one byte
and 1 microsecond and the RRC A takes two bytes and 2 microsec-
onds, the RRCA was chosen.
If the value at BYTE was 101110012, the character stored at BUF
through BUF+7 for this routine would be 31, 30, 31, 31, 31, 30, 30,
31, all hexadecimal. .
The rotate shifts may be used in conjunction with logical shifts to
facilitate multiple-precision shifts. Suppose that we wish (for some
unfathomable reason) to shift a 3-byte operand located at UDGE
two bit positions to the left in a logical shift. The rotate may be used
to propagate any carry along the 3-byte chain as follows:
SLUDGE EQU $
SHIFT LEFT UDGE
UDGE IX 2
LD
SLA
, +
(IX)
0 TO BIT 0, BIT 7 TO C
RL (IX-1)
C TO BIT 0, BIT 7 TO C
RL
(IX-2 )
C TO BIT 0, BIT 7 TO C
SLA
(IX)
0 TO BIT 0 , BIT 7 TO C
RL (IX-1)
C TO BIT 0, BIT 7 TO C
RL
(IX-2)
C TO BIT 0, BIT 7 TO C
Another variation of this implementation uses the HL register.
SLUDGE LD HL,(UDGE+I) TWO LS BYTES
LD IY, UDGE
ADD HL,HL SHIFT LEFT 1 TO C
RL IY C TO MS BYTE
ADD HL,HL SHIFT LEFT 1 TO C
RL IY C TO MS BYTE
LD (UDGE+1),HL STORE LS BYTE
ARITHMETIC SHIFTS
We have already covered the shift left arithmetic in previous ex-
amples. Various manufacturers implement arithmetic left shifts in
one of two ways. Many equate a left arithmetic shift to a left logical
and leave it go at that-the sign is simply shifted out on the first shift.
Other manufacturers retain the sign on a left shift. The bit in bit
position six is shifted out into the carry and bit seven remains.
In any event, the Shift Right Arithmetic is unambiguous. The sign
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