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.
Signed-off-by: Peter Maydell <peter.mayd...@linaro.org>
---
Specifically, I figured these out as I was going through looking
for the insns which write SP. These comments turn out not to
be relevant to those instructions, but I don't want to throw
them away.
---
target/arm/translate.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
diff --git a/target/arm/translate.c b/target/arm/translate.c
index 25a8fe672f5..fcb33b8a503 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -10623,6 +10623,10 @@ 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(tmp, op, tmp2, logic_cc);
@@ -11674,7 +11678,11 @@ 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)) {
@@ -11711,7 +11719,10 @@ 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 */
@@ -11848,7 +11859,10 @@ 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;
--
2.19.0
