diff --git a/asm/lumbda b/asm/lumbda
index 8fccc7c..6cf4790 100755
Binary files a/asm/lumbda and b/asm/lumbda differ
diff --git a/asm/lumbda-gc b/asm/lumbda-gc
index 5214f87..55eba6e 100755
Binary files a/asm/lumbda-gc and b/asm/lumbda-gc differ
diff --git a/asm/lumbda-gc.o b/asm/lumbda-gc.o
index 16533c0..29f33f3 100644
Binary files a/asm/lumbda-gc.o and b/asm/lumbda-gc.o differ
diff --git a/asm/lumbda.o b/asm/lumbda.o
index 0089363..8e1a7e1 100644
Binary files a/asm/lumbda.o and b/asm/lumbda.o differ
diff --git a/asm/lumbda.s b/asm/lumbda.s
index e873157..0d2fd50 100644
--- a/asm/lumbda.s
+++ b/asm/lumbda.s
@@ -194,15 +194,16 @@
 .equ BI_HS_SIZE,  106
 .equ BI_HS_LIST,  107
 .equ BI_TCPSENDFILE, 108
+.equ BI_ISQRT,    109
 .ifdef GC_NAIVE
-.equ BI_GC_COLLECT, 109
-.equ BI_GC_STATS,   110
-.equ BI_WITH_ARENA, 111
-.equ BI_ARENA_STATS, 112
-.equ BI_ARENA_SET_MODE, 113
-.equ BI_COUNT,      114
+.equ BI_GC_COLLECT, 110
+.equ BI_GC_STATS,   111
+.equ BI_WITH_ARENA, 112
+.equ BI_ARENA_STATS, 113
+.equ BI_ARENA_SET_MODE, 114
+.equ BI_COUNT,      115
 .else
-.equ BI_COUNT,    109
+.equ BI_COUNT,    110
 .endif
 
 # ============================================================
@@ -299,6 +300,7 @@ bn_listp:       .byte 5;  .ascii "list?"
 bn_substr:      .byte 9;  .ascii "substring"
 bn_expt:        .byte 4;  .ascii "expt"
 bn_gcd:         .byte 3;  .ascii "gcd"
+bn_isqrt:       .byte 5;  .ascii "isqrt"
 bn_integerp:    .byte 8;  .ascii "integer?"
 bn_portalsave:  .byte 11; .ascii "portal-save"
 bn_portalresume:.byte 13; .ascii "portal-resume"
@@ -388,6 +390,7 @@ bi_names:
     .quad bn_htexists, bn_htsize, bn_htkeys, bn_htvals, bn_htalist
     .quad bn_hsmake, bn_hsp, bn_hsadd, bn_hshas, bn_hssize, bn_hslist
     .quad bn_tcpsendfile
+    .quad bn_isqrt
 .ifdef GC_NAIVE
     .quad bn_gccollect, bn_gcstats, bn_witharena, bn_arenastats, bn_arenamode
 .endif
@@ -399,6 +402,8 @@ err_notproc:    .ascii "Error: not a procedure\n"
 .equ err_notproc_len, . - err_notproc
 err_oom:        .ascii "Error: out of memory\n"
 .equ err_oom_len, . - err_oom
+err_isqrt_neg:  .ascii "Error: isqrt: negative argument\n"
+.equ err_isqrt_neg_len, . - err_isqrt_neg
 
 # Print strings
 s_true:         .ascii "#t"
@@ -3355,6 +3360,8 @@ eval_list:
     je bi_expt
     cmpq $BI_GCD, %rax
     je bi_gcd
+    cmpq $BI_ISQRT, %rax
+    je bi_isqrt
     cmpq $BI_INTEGERP, %rax
     je bi_integerp
     cmpq $BI_PORTALSAVE, %rax
@@ -4663,6 +4670,58 @@ bi_gcd:
     call make_int
     RET_VAL
 
+# bi_isqrt: floor of integer square root (bit-by-bit, no FPU).
+# Negative input -> error exit. Registers: %r8 = x, %r9 = res, %r10 = bit.
+bi_isqrt:
+    GETARG %rax
+    sarq $3, %rax                # untag
+    testq %rax, %rax
+    js .bisqrt_neg
+    cmpq $2, %rax
+    jl .bisqrt_small             # v < 2 -> result is v itself
+    movq %rax, %r8               # x
+    xorq %r9, %r9                # res
+    movq $1, %r10
+    shlq $62, %r10               # bit = 1<<62
+.bisqrt_align:
+    cmpq %r8, %r10
+    jbe .bisqrt_loop
+    shrq $2, %r10
+    jmp .bisqrt_align
+.bisqrt_loop:
+    testq %r10, %r10
+    jz .bisqrt_ret
+    movq %r9, %rcx
+    addq %r10, %rcx              # rcx = res + bit
+    cmpq %rcx, %r8
+    jb .bisqrt_no
+    subq %rcx, %r8               # x -= res + bit
+    shrq $1, %r9
+    addq %r10, %r9               # res = (res>>1) + bit
+    jmp .bisqrt_next
+.bisqrt_no:
+    shrq $1, %r9                 # res >>= 1
+.bisqrt_next:
+    shrq $2, %r10                # bit >>= 2
+    jmp .bisqrt_loop
+.bisqrt_ret:
+    movq %r9, %rdi
+    call make_int
+    RET_VAL
+.bisqrt_small:
+    movq %rax, %rdi
+    call make_int
+    RET_VAL
+.bisqrt_neg:
+    movq $SYS_WRITE, %rax
+    movq $2, %rdi
+    leaq err_isqrt_neg(%rip), %rsi
+    movq $err_isqrt_neg_len, %rdx
+    syscall
+    movq $SYS_EXIT, %rax
+    movq $1, %rdi
+    syscall
+
 bi_vector:
     # (vector e1 e2 ...) — build from remaining args in %r12
     # Count args
diff --git a/asm/test.sh b/asm/test.sh
index 73c55f4..d11bfe6 100644
--- a/asm/test.sh
+++ b/asm/test.sh
@@ -52,6 +52,11 @@ check "max"        "(max 3 1 4)"       "4"
 check "expt"       "(* 2 2 2 2 2 2 2 2 2 2)"  "1024"
 check "div"        "(/ 10 2)"           "5"
 check "expt-bi"    "(expt 2 10)"        "1024"
+check "isqrt-0"    "(isqrt 0)"          "0"
+check "isqrt-1"    "(isqrt 1)"          "1"
+check "isqrt-perfect" "(isqrt 144)"     "12"
+check "isqrt-floor" "(isqrt 10)"        "3"
+check "isqrt-big"  "(isqrt 1000000000000)" "1000000"
 check "odd?"       "(odd? 3)"           "#t"
 check "odd?-even"  "(odd? 4)"           "#f"
 check "even?"      "(even? 4)"          "#t"
diff --git a/c/builtins.c b/c/builtins.c
index 16b64a6..49f5872 100644
--- a/c/builtins.c
+++ b/c/builtins.c
@@ -70,6 +70,24 @@ static Value bi_div(Value *a, int n, Env *e) {
     }
 
 MATH_1(sqrt, sqrt)
+
+static Value bi_isqrt(Value *a, int n, Env *e) {
+    (void)e; CHECK_ARITY("isqrt", 1);
+    int64_t v = as_number_int(AS_NUM(a[0]));
+    if (v < 0) lisp_error("isqrt: negative argument: %lld", (long long)v);
+    if (v < 2) return VAL_INT(v);
+    uint64_t x = (uint64_t)v;
+    uint64_t res = 0;
+    uint64_t bit = (uint64_t)1 << 62;
+    while (bit > x) bit >>= 2;
+    while (bit != 0) {
+        if (x >= res + bit) { x -= res + bit; res = (res >> 1) + bit; }
+        else { res >>= 1; }
+        bit >>= 2;
+    }
+    return VAL_INT((int64_t)res);
+}
+
 MATH_1(sin, sin)
 MATH_1(cos, cos)
 MATH_1(tan, tan)
@@ -1752,7 +1770,7 @@ Env *make_global_env(void) {
     DEF("expt", bi_expt); DEF("abs", bi_abs);
     DEF("floor", bi_floor); DEF("ceiling", bi_ceiling); DEF("round", bi_round);
     DEF("truncate", bi_truncate);
-    DEF("sqrt", bi_sqrt); DEF("log", bi_log); DEF("exp", bi_exp);
+    DEF("sqrt", bi_sqrt); DEF("isqrt", bi_isqrt); DEF("log", bi_log); DEF("exp", bi_exp);
     DEF("sin", bi_sin); DEF("cos", bi_cos); DEF("tan", bi_tan);
     DEF("asin", bi_asin); DEF("acos", bi_acos); DEF("atan", bi_atan);
     DEF("min", bi_min); DEF("max", bi_max);
diff --git a/lumbda.py b/lumbda.py
index 6cfdce1..14600d6 100644
--- a/lumbda.py
+++ b/lumbda.py
@@ -2871,6 +2871,7 @@ def make_global_env():
     d(S('floor/'),     lambda a, _: (math.floor(_num(a[0]) / _num(a[1])),
                                      _num(a[0]) - _num(a[1]) * math.floor(_num(a[0]) / _num(a[1]))))
     d(S('sqrt'),       lambda a, _: math.sqrt(_num(a[0])))
+    d(S('isqrt'),      lambda a, _: math.isqrt(int(_num(a[0]))))
     d(S('log'),        lambda a, _: math.log(_num(a[0])) if len(a) == 1 else math.log(_num(a[0]), _num(a[1])))
     d(S('exp'),        lambda a, _: math.exp(_num(a[0])))
     d(S('sin'),        lambda a, _: math.sin(_num(a[0])))
diff --git a/tests/functional.lsp b/tests/functional.lsp
index 34ac7da..8d16486 100644
--- a/tests/functional.lsp
+++ b/tests/functional.lsp
@@ -50,6 +50,13 @@
 (assert-equal "max" (max 3 1 4 1 5) 5)
 (assert-equal "modulo" (modulo 10 3) 1)
 (assert-equal "expt" (expt 2 10) 1024)
+(assert-equal "isqrt-0" (isqrt 0) 0)
+(assert-equal "isqrt-1" (isqrt 1) 1)
+(assert-equal "isqrt-perfect" (isqrt 144) 12)
+(assert-equal "isqrt-floor" (isqrt 10) 3)
+(assert-equal "isqrt-just-below" (isqrt 99) 9)
+(assert-equal "isqrt-just-above" (isqrt 101) 10)
+(assert-equal "isqrt-large" (isqrt 1000000000000) 1000000)
 
 ;;; ═══════════════════════════════════════════════════════════════
 ;;; Comparison