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.
\(\begin{rcases} A-(HL) \\ HL \longleftarrow HL+1 \text{ if } D = 0\\ HL \longleftarrow HL-1 \text{ if } D=1 \\BC \longleftarrow BC-1 \end{rcases} \text{repeat while } \begin{cases} L=1\\A \not = (HL)\\BC \not = 0 \end{cases}\)
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 0 | 1 | 1 | 0 | 1 | ED |
| 1 | 0 | 1 | L | D | 0 | 0 | 1 |
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
- Calculate difference between
Aand content of memory in(HL)to set/clear Z flag - Decrement
BCby one HLis either incremented (for CPI/CPIR) or decremented (for CPD/CPDR) by one.-
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 | Set if result is negative | ||||||||
| z | Set if A = (HL) | ||||||||
| h | Borrow from bit 4, otherwise reset | ||||||||
| p/v | Non-repeating: Set if BC-1 != 0, otherwise reset Repeating: N/A as BC=0 after instruction completes |
Opcode Matrix
| Increment | Decrement | |
|---|---|---|
| Single Search |
CPI
EDA1216 |
CPD
EDA9216 |
| Repeat Search |
CPIR
EDB1221 |
CPDR
EDB9221 |
|
Instruction
Opcode hexSize bytesCycle count
| Memory |