diff --git a/backends/cxxrtl/runtime/cxxrtl/cxxrtl.h b/backends/cxxrtl/runtime/cxxrtl/cxxrtl.h
index 008401232..31085b745 100644
--- a/backends/cxxrtl/runtime/cxxrtl/cxxrtl.h
+++ b/backends/cxxrtl/runtime/cxxrtl/cxxrtl.h
@@ -1042,6 +1042,7 @@ struct fmt_part {
 	} sign; // = MINUS;
 	bool hex_upper; // = false;
 	bool show_base; // = false;
+	bool group; // = false;
 
 	// VLOG_TIME type
 	bool realtime; // = false;
@@ -1104,13 +1105,19 @@ struct fmt_part {
 				}
 
 				if (base == 2) {
+					for (size_t index = 0; index < width; index++) {
+						if (group && index > 0 && index % 4 == 0)
+							buf += '_';
+						buf += (val.bit(index) ? '1' : '0');
+					}
 					if (show_base)
-						buf += "0b";
-					for (size_t i = width; i > 0; i--)
-						buf += (val.bit(i - 1) ? '1' : '0');
+						buf += "b0";
+					std::reverse(buf.begin(), buf.end());
 				} else if (base == 8 || base == 16) {
 					size_t step = (base == 16) ? 4 : 3;
 					for (size_t index = 0; index < width; index += step) {
+						if (group && index > 0 && index % (4 * step) == 0)
+							buf += '_';
 						uint8_t value = val.bit(index) | (val.bit(index + 1) << 1) | (val.bit(index + 2) << 2);
 						if (step == 4)
 							value |= val.bit(index + 3) << 3;
@@ -1126,7 +1133,10 @@ struct fmt_part {
 					if (val.is_zero())
 						buf += '0';
 					value<(Bits > 4 ? Bits : 4)> xval = val.template zext<(Bits > 4 ? Bits : 4)>();
+					size_t index = 0;
 					while (!xval.is_zero()) {
+						if (group && index > 0 && index % 3 == 0)
+							buf += '_';
 						value<(Bits > 4 ? Bits : 4)> quotient, remainder;
 						if (Bits >= 4)
 							std::tie(quotient, remainder) = xval.udivmod(value<(Bits > 4 ? Bits : 4)>{10u});
@@ -1134,6 +1144,7 @@ struct fmt_part {
 							std::tie(quotient, remainder) = std::make_pair(value<(Bits > 4 ? Bits : 4)>{0u}, xval);
 						buf += '0' + remainder.template trunc<4>().template get<uint8_t>();
 						xval = quotient;
+						index++;
 					}
 					if (show_base)
 						buf += "d0";
diff --git a/kernel/fmt.cc b/kernel/fmt.cc
index 8f4e61722..f5793d796 100644
--- a/kernel/fmt.cc
+++ b/kernel/fmt.cc
@@ -156,6 +156,10 @@ void Fmt::parse_rtlil(const RTLIL::Cell *cell) {
 					part.show_base = true;
 					++i;
 				}
+				if (fmt[i] == '_') {
+					part.group = true;
+					++i;
+				}
 
 				if (fmt[i] == 'u')
 					part.signed_ = false;
@@ -241,6 +245,7 @@ void Fmt::emit_rtlil(RTLIL::Cell *cell) const {
 						case FmtPart::SPACE_MINUS: fmt += ' '; break;
 					}
 					fmt += part.show_base ? "#" : "";
+					fmt += part.group ? "_" : "";
 					fmt += part.signed_ ? 's' : 'u';
 				} else if (part.type == FmtPart::STRING) {
 					fmt += 'c';
@@ -683,6 +688,7 @@ void Fmt::emit_cxxrtl(std::ostream &os, std::string indent, std::function<void(c
 		os << ", ";
 		os << part.hex_upper << ", ";
 		os << part.show_base << ", ";
+		os << part.group << ", ";
 		os << part.realtime;
 		os << " }.render(";
 		emit_sig(part.sig);
@@ -737,12 +743,27 @@ std::string Fmt::render() const
 					}
 
 					if (part.base == 2) {
+						for (size_t index = 0; index < (size_t)value.size(); index++) {
+							if (part.group && index > 0 && index % 4 == 0)
+								buf += '_';
+							RTLIL::State bit = value[index];
+							if (bit == State::Sx)
+								buf += 'x';
+							else if (bit == State::Sz)
+								buf += 'z';
+							else if (bit == State::S1)
+								buf += '1';
+							else /* if (bit == State::S0) */
+								buf += '0';
+						}
 						if (part.show_base)
-							buf += "0b";
-						buf = value.as_string();
+							buf += "b0";
+						std::reverse(buf.begin(), buf.end());
 					} else if (part.base == 8 || part.base == 16) {
 						size_t step = (part.base == 16) ? 4 : 3;
 						for (size_t index = 0; index < (size_t)value.size(); index += step) {
+							if (part.group && index > 0 && index % (4 * step) == 0)
+								buf += '_';
 							RTLIL::Const subvalue = value.extract(index, min(step, value.size() - index));
 							bool has_x = false, all_x = true, has_z = false, all_z = true;
 							for (State bit : subvalue) {
@@ -799,9 +820,13 @@ std::string Fmt::render() const
 							log_assert(absvalue.is_fully_def());
 							if (absvalue.is_fully_zero())
 								buf += '0';
+							size_t index = 0;
 							while (!absvalue.is_fully_zero())	{
+								if (part.group && index > 0 && index % 3 == 0)
+									buf += '_';
 								buf += '0' + RTLIL::const_mod(absvalue, 10, false, false, 4).as_int();
 								absvalue = RTLIL::const_div(absvalue, 10, false, false, absvalue.size());
+								index++;
 							}
 							if (part.show_base)
 								buf += "d0";
diff --git a/kernel/fmt.h b/kernel/fmt.h
index 10b1711f1..2d4b24979 100644
--- a/kernel/fmt.h
+++ b/kernel/fmt.h
@@ -85,6 +85,7 @@ struct FmtPart {
 	} sign = MINUS;
 	bool hex_upper = false;
 	bool show_base = false;
+	bool group = false;
 
 	// VLOG_TIME type
 	bool realtime = false;