UM0438 datasheet(25/50 Pages) STMICROELECTRONICS

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UM0438

Simplified mnemonics for VLE instructions

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The BO32, BI32, and BO16, BI16 operands correspond to fields in the instruction opcode,

as Figure 1 shows for Branch Conditional (e_bc, e_bcl, and se_bc) instructions.

Figure 1.

Branch conditional (e_bc, se_bc) instruction formats

e_bc (e_bcl)

se_bc

Both the BO32 and BO16 operands allow testing whether a CR bit causes a branch to occur

based on a true or false condition. The BO32 operand provides additional capability that

allows branch operations that involve decrementing the CTR and testing for a zero or non-

zero CTR value.

The BI32 and BI16 operands identify a CR bit to test (whether a comparison is less than or

greater than, for example). The simplified mnemonics avoid the need to memorize the

numerical values for BO32, BI32, and BO16, BI16 operands.

For example, e_bc 2,0,target is a conditional branch that, as a BO32 value of 2 (0b10)

indicates, decrements the CTR, then branches if the decremented CTR is not zero. The

operation specified by BO32 is abbreviated as d (for decrement) and nz (for not zero), which

replace the c in the original mnemonic; so the simplified mnemonic for e_bc becomes

e_bdnz. The branch does not depend on a condition in the CR, so BI32 can be eliminated,

reducing the expression to e_bdnz target.

In addition to CTR operations, the BO32 operand provides branch decisions based on true

or false conditions. For example, if a branch instruction depends on an equal condition in

CR0, the expression is e_bc 1,2,target. To specify a true condition, the BO32 value

becomes 1; the CR0 equal field is indicated by a BI32 value of 2. Incorporating the branch-

if-true condition a ‘t’ is used to replace the c in the original mnemonic, e_bt. The BI32 value

of 2 is replaced by the eq symbol. Using the simplified mnemonic and the eq operand, the

expression becomes e_bt eq,target.

This example tests CR0[EQ]; however, to test the equal condition in CR3 (CR bit 14), the

expression becomes e_bc 1,14,target. The BI32 operand of 14 indicates CR[14] (CR3[2], or

Table 13.

Branch instructions

Instruction name

Mnemonic

Syntax

Branch

e_b (e_bl)

se_b (se_bl)

target_addr

Branch Conditional

e_bc (e_bcl)

se_bc

BO32,BI32,target_addr

BO16,BI16,target_addr

Branch to Link Register

se_blr (se_blrl)

—

Branch to Count Register

se_bctr (se_bctrl)

—

9 10 11 12

15 16

30 31

0 1 1 1 1 0 1 0 0 0 BO32

BI32

BD15

1111

0 BO16 BI16

BD8

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