diff --git a/desk.acc/a2d.inc b/desk.acc/a2d.inc
index bce7d1f..83dcdda 100644
--- a/desk.acc/a2d.inc
+++ b/desk.acc/a2d.inc
@@ -174,9 +174,9 @@ A2D_DRAW_TEXT   := $19          ; Drawn at last SET_POS as left, baseline
         ;; .addr data
         ;; .byte length
 
-A2D_UNK_1A      := $1A          ; Unknown - used in calculator before window creation,
-                                ; and after destruction
+A2D_CONFIGURE_ZP_USE    := $1A  ; Configure ZP usage by API (speed vs. convenience)
         ;; (input length 1 byte)
+        ;; .byte flag           (AWS_CZP_*; high bit set = preserve ZP during calls)
 
 A2D_SET_CURSOR  := $24          ; Change cursor pattern
         ;; (input not copied)
@@ -394,6 +394,10 @@ A2D_CWS_SCROLL_THUMB   := 1 << 6
 A2D_CWS_SCROLL_TRACK   := 1 << 0
 A2D_CWS_SCROLL_NORMAL  := A2D_CWS_SCROLL_ENABLED | A2D_CWS_SCROLL_THUMB | A2D_CWS_SCROLL_TRACK
 
+;;; Used in A2D_CONFIGURE_ZP_USE
+A2D_CZP_PRESERVE  := 1<<7
+A2D_CZP_OVERWRITE := 0
+
 ;;; Used in various state blocks
 A2D_DEFAULT_MSKAND := $FF
 A2D_DEFAULT_MSKOR  := $00
diff --git a/desk.acc/calc_fixed.s b/desk.acc/calc_fixed.s
index 4cc87ea..9402c87 100644
--- a/desk.acc/calc_fixed.s
+++ b/desk.acc/calc_fixed.s
@@ -198,11 +198,13 @@ id:     .byte   0
 .endproc
         query_state_params_id := query_state_params::id
 
-        ;; param block for a 1A call
-L08D4:  .byte   $80
+.proc preserve_zp_params
+flag:  .byte   A2D_CZP_PRESERVE
+.endproc
 
-        ;; param block for a 1A call
-L08D5:  .byte   $00
+.proc overwrite_zp_params
+flag:  .byte   A2D_CZP_OVERWRITE
+.endproc
 
 ;;; ==================================================
 ;;; Button Definitions
@@ -760,10 +762,10 @@ title:  PASCAL_STRING "Calc"
 init:   sta     ALTZPON
         lda     LCBANK1
         lda     LCBANK1
-        A2D_CALL $1A, L08D4     ; if NOP'd, display renders like a bar code later
+        A2D_CALL A2D_CONFIGURE_ZP_USE, preserve_zp_params
         A2D_CALL A2D_CREATE_WINDOW, create_window_params
         A2D_CALL A2D_QUERY_SCREEN, state_params
-        A2D_CALL A2D_SET_STATE, state_params     ; set clipping bounds?
+        A2D_CALL A2D_SET_STATE, state_params
         A2D_CALL $2B                          ; reset drawing state?
 
         jsr     reset_buffer2
@@ -870,7 +872,7 @@ ignore_click:
 exit:   A2D_CALL A2D_DESTROY_WINDOW, destroy_window_params
         DESKTOP_CALL DESKTOP_REDRAW_ICONS
         lda     ROMIN2
-        A2D_CALL $1A, L08D5     ; ??? one byte input value?
+        A2D_CALL A2D_CONFIGURE_ZP_USE, overwrite_zp_params
 
 .proc do_close
         ;; Copy following routine to ZP and invoke it
@@ -1045,7 +1047,7 @@ miss:   clc
         sec
         rts
 
-:       cpx     #col4_left-border_lt             ; col 4?
+:       cpx     #col4_left-border_lt            ; col 4?
         bcc     miss
         cpx     #col4_right+border_br - 1       ; bug in original?
         bcs     miss
diff --git a/desk.acc/calculator.s b/desk.acc/calculator.s
index bc326f5..1a68106 100644
--- a/desk.acc/calculator.s
+++ b/desk.acc/calculator.s
@@ -203,11 +203,13 @@ id:     .byte   0
 .endproc
         query_state_params_id := query_state_params::id
 
-        ;; param block for a 1A call
-L08D4:  .byte   $80
+.proc preserve_zp_params
+flag:   .byte   A2D_CZP_PRESERVE
+.endproc
 
-        ;; param block for a 1A call
-L08D5:  .byte   $00
+.proc overwrite_zp_params
+flag:   .byte   A2D_CZP_OVERWRITE
+.endproc
 
 ;;; ==================================================
 ;;; Button Definitions
@@ -793,7 +795,7 @@ cursor: .byte   px(%0000000),px(%0000000) ; cursor
 init:   sta     ALTZPON
         lda     LCBANK1
         lda     LCBANK1
-        A2D_CALL $1A, L08D4     ; if NOP'd, display renders like a bar code later
+        A2D_CALL A2D_CONFIGURE_ZP_USE, preserve_zp_params
         A2D_CALL A2D_CREATE_WINDOW, create_window_params
         A2D_CALL A2D_QUERY_SCREEN, state_params
         A2D_CALL A2D_SET_STATE, state_params     ; set clipping bounds?
@@ -915,7 +917,7 @@ exit:   lda     LCBANK1
         A2D_CALL A2D_DESTROY_WINDOW, destroy_window_params
         DESKTOP_CALL DESKTOP_REDRAW_ICONS
         lda     ROMIN2
-        A2D_CALL $1A, L08D5     ; ??? one byte input value?
+        A2D_CALL A2D_CONFIGURE_ZP_USE, overwrite_zp_params
 
 .proc do_close
         ;; Copy following routine to ZP and invoke it