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target/arm: Add some comments in Thumb decode

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Add some comments to the Thumb decoder indicating what bits
of the instruction have been decoded at various points in
the code.

This is not an exhaustive set of comments; we're gradually
adding comments as we work with particular bits of the code.

Backports commit a2d12f0f34e9c5ef8a193556fde983aa186fa73a from qemu
This commit is contained in:
Peter Maydell 2018-10-08 14:07:00 -04:00 committed by Lioncash
parent ca5d7b8fd2
commit 0fc6e2c183
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GPG key ID: 4E3C3CC1031BA9C7
1 changed files with 17 additions and 3 deletions
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20
qemu/target/arm/translate.c
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@ -10811,6 +10811,10 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
tmp2 = load_reg(s, rm);
if ((insn & 0x70) != 0)
goto illegal_op;
/*
* 0b1111_1010_0xxx_xxxx_1111_xxxx_0000_xxxx:
* - MOV, MOVS (register-shifted register), flagsetting
*/
op = (insn >> 21) & 3;
logic_cc = (insn & (1 << 20)) != 0;
gen_arm_shift_reg(s, tmp, op, tmp2, logic_cc);
@ -11889,7 +11893,11 @@ static void disas_thumb_insn(DisasContext *s, uint32_t insn)
rd = insn & 7;
op = (insn >> 11) & 3;
if (op == 3) {
/* add/subtract */
/*
* 0b0001_1xxx_xxxx_xxxx
* - Add, subtract (three low registers)
* - Add, subtract (two low registers and immediate)
*/
rn = (insn >> 3) & 7;
tmp = load_reg(s, rn);
if (insn & (1 << 10)) {
@ -11926,7 +11934,10 @@ static void disas_thumb_insn(DisasContext *s, uint32_t insn)
}
break;
case 2: case 3:
/* arithmetic large immediate */
/*
* 0b001x_xxxx_xxxx_xxxx
* - Add, subtract, compare, move (one low register and immediate)
*/
op = (insn >> 11) & 3;
rd = (insn >> 8) & 0x7;
if (op == 0) { /* mov */
@ -12063,7 +12074,10 @@ static void disas_thumb_insn(DisasContext *s, uint32_t insn)
break;
}
/* data processing register */
/*
* 0b0100_00xx_xxxx_xxxx
* - Data-processing (two low registers)
*/
rd = insn & 7;
rm = (insn >> 3) & 7;
op = (insn >> 6) & 0xf;