PM0044 |
STM8 addressing modes |
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6.12Relative Direct addressing mode
Table 35. |
Overview of Relative Direct addressing mode instructions |
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Addressing mode |
Syntax |
EA formula |
Ptr Adr |
Ptr Size |
Dest adr |
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Direct |
Relative |
off |
PC = PC + off |
op + 1 |
--- |
PC +127/-128 |
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This addressing mode is used to modify the PC register value, by adding an 8-bit signed offset to it. The offset added to the PC register value is relative to the start of the next instruction.
Table 36. Available Relative Direct instructions
Instructions |
Functions |
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JRxx |
Conditional Jump |
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JRA |
Jump Relative Always |
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CALLR |
Call Relative |
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The offset follows the op-code.
Example: |
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04A7 |
2717 |
jreq |
skip |
04A9 |
9D |
nop |
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04AA |
9D |
nop |
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04C0 |
20FE |
skip jra* |
; Infinite loop |
Action:
if (Z == 1)then PC = PC + $17 = $04A9 + $17 = $04C0 elsePC = PC= $04A9
Doc ID 13590 Rev 3 |
55/162 |
STM8 addressing modes |
PM0044 |
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Figure 21. Relative Direct addressing mode example
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Before completion |
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CC |
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Z |
JREQ SKIP |
27 |
04A7 |
PC |
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17 |
04A8 |
04A7 |
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04A9 |
02 |
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04A7 |
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Adder |
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04A9 |
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EA |
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After completion |
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(Branch taken) |
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CC |
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Z = 1 |
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PC |
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JREQ SKIP |
27 |
04A7 |
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04A9 |
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1704A8
04A9
17 
Adder
SKIP : |
04C0 |
04C0 |
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New PC
04C0
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After completion |
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(No branch taken) |
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CC |
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Z = 0 |
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JREQ SKIP |
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27 |
04A7 |
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New PC |
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17 |
04A8 |
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04A9 |
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04A9 |
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Steps to Determine
Effective Address
PC = 04A7
PC = PC + 1 = 04A8 TEMP = (PC) = 17 PC = PC +1 = 04A9 Stop here if there
is no Branch; i.e., Z = 0 EA = PC + TEMP
=04A9 + 17
=04C0
New PC = EA if Branch is taken
Instruction Complete
New PC = EA = 04C0
04A9
EA
Instruction Complete
New PC = EA = 04A9
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Doc ID 13590 Rev 3 |