Branch On Coprocessor Condition Instruction - Motorola MC68020 User Manual

7.2.2.1 BRANCH ON COPROCESSOR CONDITION INSTRUCTION. The conditional
instruction category includes two formats of the M68000 family branch instruction. These
instructions branch on conditions related to the coprocessor operation. They execute
similarly to the conditional branch instructions provided in the M68000 family instruction
set.
7.2.2.1.1 Format. Figure 7-9 shows the format of the branch on coprocessor condition
instruction that provides a word-length displacement. Figure 7-10 shows the format of this
instruction that includes a long-word displacement.
15 14 13
1 1
15 14 13
1 1
The first word of the branch on coprocessor condition instruction is the F-line operation
word. Bits 15–12 = 1111 and bits 11–9 contain the CpID code of the coprocessor that is to
evaluate the condition. The value in bits 8–6 identifies either the word or the long-word
displacement format of the branch instruction, which is specified by the cpBcc.W or
cpBcc.L mnemonic, respectively. Bits 5–0 of the F-line operation word contain the
coprocessor condition selector field. The MC68020/EC020 writes the entire operation
word to the condition CIR to initiate execution of the branch instruction by the
coprocessor. The coprocessor uses bits 5–0 to determine which condition to evaluate.
If the coprocessor requires additional information to evaluate the condition, the branch
instruction format can include this information in extension words. Following the F-line
operation word, the number of extension words is determined by the coprocessor design.
The final word(s) of the cpBcc instruction format contains the displacement used by the
main processor to calculate the destination address when the branch is taken.
7.2.2.1.2 Protocol. Figure 7-8 shows the protocol for the cpBcc.L and cpBcc.W
instructions. The main processor initiates the instruction by writing the F-line operation
word to the condition CIR to transfer the condition selector to the coprocessor. The main
7-12
12 11 9
1 1 CpID
OPTIONAL COPROCESSOR-DEFINED EXTENSION WORDS
Figure 7-9. Branch on Coprocessor Condition
Instruction Format (cpBcc.W)
12 11 9
1 1 CpID
OPTIONAL COPROCESSOR-DEFINED EXTENSION WORDS
DISPLACEMENT — HIGH
DISPLACEMENT — LOW
Figure 7-10. Branch on Coprocessor Condition
Instruction Format (cpBcc.L)
M68020 USER'S MANUAL
8 7 6 5
0 1 0
DISPLACEMENT
8 7 6 5
0 1 1
0
CONDITION SELECTOR
0
CONDITION SELECTOR
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