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data processing pc 12 bytes ahead if register shift

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Matthew Berry 2020-11-01 13:09:17 -08:00
parent ec08f0ee21
commit 184e7853da
1 changed files with 6 additions and 0 deletions
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6
src/crab/arm/data_processing.cr
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@ -5,6 +5,11 @@ module ARM
set_conditions = bit?(instr, 20)
rn = bits(instr, 16..19)
rd = bits(instr, 12..15)
# The PC value will be the address of the instruction, plus 8 or 12 bytes due to instruction
# prefetching. If the shift amount is specified in the instruction, the PC will be 8 bytes
# ahead. If a register is used to specify the shift amount the PC will be 12 bytes ahead.
pc_reads_12_ahead = !imm_flag && bit?(instr, 4)
@r[15] &+= 4 if pc_reads_12_ahead
operand_2 = if imm_flag # Operand 2 is an immediate
immediate_offset bits(instr, 0..11), set_conditions
else # Operand 2 is a register
@ -40,5 +45,6 @@ module ARM
@cpsr.zero = res == 0
@cpsr.negative = bit?(res, 31)
end
@r[15] &-= 4 if pc_reads_12_ahead
end
end