Motorola MC68020 User Manual page 169

The final portion of the cpScc instruction format contains zero to five effective address
extension words. These words contain any additional information required to calculate the
effective address specified by bits 5–0 of the F-line operation word.
7.2.2.2.2 Protocol. Figure 7-8 shows the protocol for the cpScc instruction. The
MC68020/EC020 transfers the condition selector to the coprocessor by writing the word
following the F-line operation word to the condition CIR. The main processor then reads
the response CIR to determine its next action. The coprocessor can return a response
primitive to request services necessary to evaluate the condition. The operation of the
cpScc instruction depends on the condition evaluation indicator returned to the main
processor by the coprocessor. When the coprocessor returns the false condition indicator,
the main processor evaluates the effective address specified by bits 5–0 of the F-line
operation word and sets the byte at that effective address to FALSE (all bits cleared).
When the coprocessor returns the true condition indicator, the main processor sets the
byte at the effective address to TRUE (all bits set to one).
7.2.2.3 TEST COPROCESSOR CONDITION, DECREMENT, AND BRANCH
INSTRUCTION. The operation of the test coprocessor condition, decrement, and branch
instruction is similar to that of the DBcc instruction provided in the M68000 family
instruction set. This operation uses a coprocessor-evaluated condition and a loop counter
in the main processor. It is useful for implementing DO UNTIL constructs used in many
high-level languages.
7.2.2.3.1 Format. Figure 7-12 shows the format of the test coprocessor condition,
decrement, and branch instruction, denoted by the cpDBcc mnemonic.
15 14 13
1 1
Figure 7-12. Test Coprocessor Condition, Decrement, and Branch
The first word of the cpDBcc instruction, F-line operation word, contains the CpID field in
bits 11–9 and 001001 in bits 8–3 to identify the cpDBcc instruction. Bits 2–0 of this
operation word specify the main processor data register used as the loop counter during
the execution of the instruction.
The second word of the cpDBcc instruction format contains the coprocessor condition
selector field in bits 5–0 and should contain zeros in bits 15–6 (reserved by Motorola) to
maintain compatibility with future M68000 products. This word is written to the condition
CIR to initiate execution of the cpDBcc instruction.
MOTOROLA
12 11 9
1 1 CpID
(RESERVED)
OPTIONAL COPROCESSOR-DEFINED EXTENSION WORDS
Instruction Format (cpDBcc)
M68020 USER'S MANUAL
8 7 6 5 5
0 0 1 0
DISPLACEMENT
4 3 2 0
0 1 REGISTER
CONDITION SELECTOR
7- 15