# Block Search of memory

Search block of memory

The Block compare instructions allow for data to be searched for in memory. The programmer needs to configure the following registers to define the properties of the search: HL is the source address to search from; BC is the number of bytes to search. A is set to the value to search for.

$$\begin{rcases} A-(HL) \\ HL \longleftarrow HL+1 \text{ if } D = 0\\ HL \longleftarrow HL-1 \text{ if } D=1 \\BC \longleftarrow BC-1 \end{rcases} \text{repeat while } \begin{cases} L=1\\A \not = (HL)\\BC \not = 0 \end{cases}$$

76543210
11101101ED
101LD001

D 0=Increment, 1=Decrement HL after each iteration.

L If set then if $$BC \not = 0$$ at the end if the instruction then $$PC \longleftarrow PC - 2$$ so that the instruction is repeated.
If BC=0 at start of a repeatable instruction then 65536 iterations will occur.

The CP* instructions compare memory against the Accumulator

1. Calculate difference between A and content of memory in (HL) to set/clear Z flag
2. Decrement BC by one
3. HL is either incremented (for CPI/CPIR) or decremented (for CPD/CPDR) by one.
4. The CPIR and CPDR instructions will loop back to step one if $$A-(HL) \not = 0 \And BC \not = 0$$
If the value was found them HL will be set to the byte after or before it depending on the direction being used.

## Timing

For the non-repeating instructions, they take 16(4,4,3,5) T-States to execute.

For the repeating instructions, they take either 21(4,4,3,5,5) T-States when they loop and 16(4,4,3,5) T-States when terminating.

Also note, that for these instructions the timing is for each iteration, not for the entire run. So if LDIR is run with BC=4 then the number of T-States for the entire operation would take 79(21+21+21+16) T-States.

##### Flags Affected
Flags
 s z - h - p/v - -
sSet if result is negative
zSet if A = (HL)
hBorrow from bit 4, otherwise reset
p/vNon-repeating: Set if BC-1 != 0, otherwise reset
Repeating: N/A as BC=0 after instruction completes
##### Opcode Matrix
IncrementDecrement
Single Search
CPI
EDA1216
CPD
EDA9216
Repeat Search
CPIR
EDB1221
CPDR
EDB9221
 Instruction Opcode hexSize bytesCycle count Memory