On Mon, 17 Aug 2026, 54weasels wrote:
High level description:
The 68881/68882 FPUs require fsave/frestore instructions to read/write state
frames. QEMU only fully implemented this for the 68040. The Sun-3 uses a 68881
FPU, and the boot ROM executes `fsave` to probe FPU presence. This patch adds
basic 68881 FPU state frame handling (NULL frame) to accurately simulate an
idle FPU state.
Impact on existing functionality:
Fixes FPU detection for 68881/68882 across all M68k boards without affecting
68040 specific state logic.
Comments such as the next Context: should go below the --- as everything
before that is part of the commit message. In QEMU a Context: tag is not
commonly used so I think you want this as a comment not part of the
commit message.
Context: This patch was originally submitted as part of the monolithic Sun-3
Machine Emulation series
(https://patchew.org/QEMU/[email protected]/) and has
been split into atomic components.
---
target/m68k/translate.c | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/target/m68k/translate.c b/target/m68k/translate.c
index 138c89d3e5..bdf619883c 100644
--- a/target/m68k/translate.c
+++ b/target/m68k/translate.c
@@ -5364,11 +5364,19 @@ DISAS_INSN(frestore)
gen_exception(s, s->base.pc_next, EXCP_PRIVILEGE);
return;
}
- if (m68k_feature(s->env, M68K_FEATURE_M68040)) {
+ if (m68k_feature(s->env, M68K_FEATURE_M68040) ||
+ m68k_feature(s->env, M68K_FEATURE_FPU)) {
M68020+ (including M68040) has FPU set so no need to keep that test and
can be replaced with M68K_FEATURE_FPU.
SRC_EA(env, addr, OS_LONG, 0, NULL);
- /* FIXME: check the state frame */
+ if (m68k_feature(s->env, M68K_FEATURE_M68040)) {
+ /* FIXME: check the state frame */
+ } else {
What about M68060 or other CPUs with built-in FPU? As none of these do
anything maybe the if/else is not needed and can just keep the FIXME
comment?
+ /*
+ * 68881/68882 FRESTORE: read the state frame
+ * (NULL frame is 4 bytes)
+ */
+ }
} else {
- disas_undef(env, s, insn);
+ disas_undef_fpu(env, s, insn);
}
}
@@ -5383,8 +5391,12 @@ DISAS_INSN(fsave)
/* always write IDLE */
TCGv idle = tcg_constant_i32(0x41000000);
DEST_EA(env, insn, OS_LONG, idle, NULL);
+ } else if (m68k_feature(s->env, M68K_FEATURE_FPU)) {
+ /* 68881/68882 FSAVE: always write NULL frame */
+ TCGv null_frame = tcg_constant_i32(0x00000000);
+ DEST_EA(env, insn, OS_LONG, null_frame, NULL);
Same comment as above about CPUs with FPU. Maybe there should be only one
block for M68K_FEATURE_FPU replacing M68K_FEATURE_M68040 and a switch for
setting the frame value?
Regards,
BALATON Zoltan
} else {
- disas_undef(env, s, insn);
+ disas_undef_fpu(env, s, insn);
}
}
#endif
