PM0044 |
STM8 addressing modes |
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6.14Bit Direct (Long) Relative addressing mode
Table 39. |
Overview of Bit Direct (Long) Relative addressing mode |
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Addressing mode |
Syntax |
EA formula |
Ptr Adr |
Ptr Size |
Dest adr |
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Long |
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(longmem) |
op + 1..2 |
Word |
0000..FFFF |
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Relative |
longmem, #pos, off |
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PC +127/- |
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Direct |
PC = PC + off |
op + 3 |
Byte |
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128 |
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This addressing mode is a combination between the Bit Direct addressing mode (for data addressing) and Relative Direct mode (for PC computation).
The data byte required for the operation is found by its memory address, which follows the op-code. The bit used for the test operation is selected by the bit selector which is encoded in the instruction op-code. Following the logical test operation, the PC register value can be modified, by adding an 8-bit signed offset to it.
Table 40. Available Bit Direct Relative instructions
Instructions |
Functions |
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BTJT, BTJF |
Bit Test and Jump |
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The data address is a word, thus allowing 0000 to FFFF addressing space (requires 2 bytes after the op-code). The bit selector #n (n=0 to 7) selects the nth bit from the byte pointed to by the address. The offset follows the op-code and data address.
Example: |
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104B |
00 |
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DRA |
dc.b |
$00 ; Port A data |
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register (input |
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value) |
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bit0 |
equ |
$0 ; data bit 0 |
04A7 |
7201104BFB wait_1 |
BTJF |
DRA, bit0, wait_1 |
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04AC |
.... |
cont_0 |
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Action:
Test = select_bit(0, ($4B)) = select_bit(0, DRA)
if (Test /= 1) |
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PC = PC + $FB |
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$0004AC - $05 = |
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$0004A7 |
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else |
PC = PC |
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$0004AC |
Doc ID 13590 Rev 3 |
59/162 |
STM8 addressing modes |
PM0044 |
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Figure 23. Bit Long Direct Relative addressing mode example
DRA .byte |
DRA |
b0 |
DRA.b0 =? 0 |
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PC |
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104C |
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04A7 |
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05 |
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04A7 |
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wait_1 |
72 |
04A7 |
Adder |
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BTJF DRA, #0, wait_1 |
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01 |
04A8 |
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10 |
04A9 |
04AC |
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4B |
04AA |
EA |
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FB |
04AB |
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After completion |
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(Branch taken) |
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(EA) |
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b0 = 0 |
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wait_1 |
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04A7 |
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72 |
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PC |
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BTJF DRA, #0, wait_1 |
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04A7 |
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01 |
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04A8 |
New PC |
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04AC |
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10 |
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04A9 |
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4B |
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04AA |
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FB |
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04AB |
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FB |
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Adder
Steps to Determine
Effective Address
PC = 04A7
PC = PC + 2 = 04A9
EA = (PC):(PC+1) = 104B Test = (EA).b0
PC = PC + 2 = 04AB
TEMP = (PC) = FC
PC = PC +1 = 04AC Stop here if there
is no Branch; i.e., Test = TRUE (1) EA = PC + TEMP
=04AA + FD
=04A7
New PC = EA if Branch is taken
Instruction Complete
New PC = EA = 04A7
04AC
wait_1
BTJF DRA, #0, wait_1
04A7 |
EA |
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After completion
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(No branch taken) |
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(EA) |
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b0 = 1 |
Instruction Complete |
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04A7 |
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New PC = EA = 04AC |
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01 |
04A8 |
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10 |
04A9 |
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4B |
04AA |
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FB |
04AB |
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New PC |
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04AC |
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04AC |
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Doc ID 13590 Rev 3 |