From 85f921a17263d8688f8cf8dbe7d58611c8b863d4 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Tue, 14 Apr 2026 19:43:03 +0200
Subject: [PATCH] Update tan_FAC1

---
 STATUS.md                            |  2 +-
 src/aliases/,aliases_ram_lowmem.s    |  3 ++-
 src/basic/math_trans/e2b4.tan_FAC1.s | 40 +++++++++++++++++++---------
 tools/similarity.c                   |  5 ++--
 4 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/STATUS.md b/STATUS.md
index 4bf4dc02..e9093411 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -264,7 +264,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$E0BE`   | `rnd_generate`         | DONE     |                                                    |
 | `$E264`   | `cos_FAC1`             | DONE     | From the Microsoft BASIC, original name: COS       |
 | `$E26B`   | `sin_FAC1`             | DONE     | From the Microsoft BASIC, original name: SIN       |
-| `$E2B4`   | `tan_FAC1`             | PARTIAL  | not fully tested yet                               |
+| `$E2B4`   | `tan_FAC1`             | DONE     | From the Microsoft BASIC, original name: TAN       |
 | `$E30E`   | `atn_FAC1`             | DONE     | From the Microsoft BASIC, original name: ATN       |
 
 <br />
diff --git a/src/aliases/,aliases_ram_lowmem.s b/src/aliases/,aliases_ram_lowmem.s
index d0fb307e..d221f916 100644
--- a/src/aliases/,aliases_ram_lowmem.s
+++ b/src/aliases/,aliases_ram_lowmem.s
@@ -53,7 +53,8 @@
 	!addr FORPNT    = $49 ; $49-$4A  -- NOT IMPLEMENTED --
 	!addr OPPTR     = $4B ; $4B-$4C  helper variable for expression computation, [!] our usage details are different
 	!addr OPMASK    = $4D ;          -- NOT IMPLEMENTED --
-	!addr DEFPNT    = $4E ; $4E-$4F  -- NOT IMPLEMENTED --
+	!addr DEFPNT    = $4E ; $4E-$4F
+	!addr TEMPF3    = DEFPNT
 	!addr DSCPNT    = $50 ; $50-$52  temporary area fro string handling, [!] our usage might differ
 	!addr FOUR6     = $53 ;          size of variable content (float = 5, integer = 2, string descriptor = 3)
 	!addr JMPER     = $54 ; $54-$56  -- NOT IMPLEMENTED --
diff --git a/src/basic/math_trans/e2b4.tan_FAC1.s b/src/basic/math_trans/e2b4.tan_FAC1.s
index db9277ee..59c52d72 100644
--- a/src/basic/math_trans/e2b4.tan_FAC1.s
+++ b/src/basic/math_trans/e2b4.tan_FAC1.s
@@ -2,9 +2,14 @@
 ;; #LAYOUT# *   BASIC_0 #TAKE
 ;; #LAYOUT# *   *       #IGNORE
 
-;
+; This file is under the MIT license, it contains code released by Microsoft Corporation.
+; See LICENSE for more information.
+
 ; Math package - tangent of FAC1 in radians
 ;
+; This is verified to be almost identical to the original Microsoft implementation where it was named TAN.
+; The difference is the optimization of the jsr to mov_r_FAC1_MEM
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 210
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
@@ -18,17 +23,28 @@
 
 
 tan_FAC1:
-    jsr mov_r_FAC1_TMP1         ; TMP1 = x
-    jsr sin_FAC1                ; FAC1 = sin(x)
-    jsr mov_r_FAC1_TMP2         ; TMP2 = sin(x)
-    lda #TEMPF1
-    ldy #0
-    jsr mov_MEM_FAC1            ; FAC1 = x
-    jsr COS                     ; FAC1 = cos(x)
-    lda #TEMPF2
-    ldy #0
-    jsr mov_MEM_FAC2            ; FAC2 = sin(x)
-    jmp div_FAC2_FAC1           ; FAC1 = tan(x)
+
+        jsr mov_r_FAC1_TMP1         ; Mov FAC1 into temporary
+        lda #$00
+        sta TANSGN                  ; Remember whether to negate
+        jsr sin_FAC1                ; Compute the sin
+        ldx #<TEMPF3
+        ldy #>TEMPF3
+        jsr mov_r_FAC1_MEM          ; In the original BASIC this does jsr GMOVMF that jmps to mov_r_FAC1_MEM
+        lda #<TEMPF1
+        ldy #>TEMPF1
+        jsr mov_MEM_FAC1            ; Put this memory loc into FAC1
+        lda #$00
+        sta FAC1_sign               ; Start off positive
+        lda TANSGN
+        jsr COSC                    ; Compute cosine
+        lda #<TEMPF3                ; Address of sine value
+        ldy #>TEMPF3
+GFDIV:
+        jmp div_MEM_FAC1            ; Divide sine by cosine and return
+COSC:
+        pha
+        jmp SIN1
 
 
 } else {
diff --git a/tools/similarity.c b/tools/similarity.c
index 7d4ba857..e2144418 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,7 +20,7 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 11
+#define NUM_CHUNKS 12
 // Pairs of start, end memory addresses of known exact similarity with the open source Microsoft BASIC
 unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB391, 0xB39D,
@@ -32,7 +32,8 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xBDB3, 0xBDC1,
     0xBDDD, 0xBF51,
     0xBFB4, 0xBFEC,
-    0xE264, 0xE2B3,
+    0xE264, 0xE2C2,     // Two byte gap here is an optimization
+    0xE2C5, 0xE2DF,
     0xE2EF, 0xE37A,
 };