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functions_immediate.go
48 lines (40 loc) · 1.24 KB
/
functions_immediate.go
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// This file is part of Gopher2600.
//
// Gopher2600 is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Gopher2600 is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Gopher2600. If not, see <https://www.gnu.org/licenses/>.
package fpu
func (fpu *FPU) VFPExpandImm(imm8 uint8, N int) uint64 {
// on pages A6-166 of "ARMv7-M"
var E int
switch N {
case 32:
E = 8
case 64:
E = 11
default:
panic("unsupported number of bits in VFPExpandImm()")
}
F := N - E - 1
imm64 := uint64(imm8)
sign := (imm64 & 0x80) >> 7
bit6 := (imm64 & 0x40) >> 6
expA := (^bit6) & 0b01
expB := uint64(0)
if bit6 == 0b01 {
expB = (bit6 << (E - 3)) - 1
}
expC := (imm64 & 0x30) >> 4
exp := (expA << (E - 1)) | (expB << 2) | expC
frac := (imm64 & 0x0f) << (F - 4)
return (sign << (E + F)) | (exp << F) | frac
}