NES: Reduce zeropage usage by merging flag for oam validity and forced blanking to one byte (#753)

gbdk-lib/libc/targets/mos6502/nes/crt0.s

@@ -66,12 +66,11 @@ __current_bank::                        .ds 1
 _sys_time::                             .ds 2
 _shadow_PPUCTRL::                       .ds 1
 _shadow_PPUMASK::                       .ds 1
-__crt0_spritePageValid:                 .ds 1
 _bkg_scroll_x::                         .ds 1
 _bkg_scroll_y::                         .ds 1
 _attribute_row_dirty::                  .ds 1
 _attribute_column_dirty::               .ds 1
-.crt0_forced_blanking::                 .ds 1
+__oam_valid_display_on::                .ds 1
 __SYSTEM::                              .ds 1
 __hblank_writes_index:                  .ds 1
 
@@ -134,7 +133,7 @@ ProcessDrawList_DoOneTransfer:
 
 ; .bndry 0x100 (skip alignment as previous alignment means page-cross won't happen)
 __crt0_doSpriteDMA:
-    bit *__crt0_spritePageValid
+    bit *__oam_valid_display_on
     bpl __crt0_doSpriteDMA_spritePageInvalid
     lda #0                      ; +2
     sta OAMADDR                 ; +4
@@ -542,8 +541,9 @@ _wait_vbl_done_waitForNextFrame:
     sty *__hblank_writes_index
 
     ; Enable OAM DMA in next NMI
-    sec
-    ror *__crt0_spritePageValid
+    lda *__oam_valid_display_on
+    ora #OAM_VALID_MASK
+    sta *__oam_valid_display_on
     ; Restore shadow registers
     pla
     sta *_bkg_scroll_y
@@ -560,8 +560,9 @@ _wait_vbl_done_waitForNextFrame_loop:
     beq _wait_vbl_done_waitForNextFrame_loop
 
     ; Disable OAM DMA in next NMI
-    clc
-    ror *__crt0_spritePageValid
+    lda *__oam_valid_display_on
+    and #~OAM_VALID_MASK
+    sta *__oam_valid_display_on
 
     rts
 
@@ -594,8 +595,9 @@ _display_off::
     sta *_shadow_PPUMASK
     sta PPUMASK
     ; Set forced blanking bit
-    sec
-    ror *.crt0_forced_blanking
+    lda *__oam_valid_display_on
+    ora #DISPLAY_OFF_MASK
+    sta *__oam_valid_display_on
     rts
 
 .display_on::
@@ -604,8 +606,9 @@ _display_on::
     ora #(PPUMASK_SHOW_BG | PPUMASK_SHOW_SPR)
     sta *_shadow_PPUMASK
     ; Clear forced blanking bit
-    clc
-    ror *.crt0_forced_blanking
+    lda *__oam_valid_display_on
+    and #~DISPLAY_OFF_MASK
+    sta *__oam_valid_display_on
     rts
 
 __crt0_RESET:
@@ -669,7 +672,7 @@ __crt0_RESET_bankSwitchValue:
     lda #(PPUMASK_SHOW_BG | PPUMASK_SHOW_SPR | PPUMASK_SHOW_BG_LC | PPUMASK_SHOW_SPR_LC)
     sta *_shadow_PPUMASK
     lda #0x80
-    sta *__crt0_spritePageValid
+    sta *__oam_valid_display_on
     ; enable NMI
     lda #(PPUCTRL_NMI | PPUCTRL_SPR_CHR)
     sta *_shadow_PPUCTRL

gbdk-lib/libc/targets/mos6502/nes/global.s

@@ -8,6 +8,10 @@
         ;; Number of 8-cycles available each frame for transfer buffer
         VRAM_DELAY_CYCLES_X8  = 171
 
+        ; Bits for quick checking of OAM validity and display ON/OFF
+        OAM_VALID_MASK      = 0x80
+        DISPLAY_OFF_MASK    = 0x40
+
         ;;  Keypad
         .UP             = 0x08
         .DOWN           = 0x04

gbdk-lib/libc/targets/mos6502/nes/set_data.s

@@ -42,8 +42,8 @@ _set_sprite_data::
     rol
     asl *.ppu_addr_lo
     rol
-    bit *.crt0_forced_blanking
-    bpl .copy_tiles_indirect
+    bit *__oam_valid_display_on
+    bvc .copy_tiles_indirect
     ; Direct write
     sta PPUADDR
     lda *.ppu_addr_lo

gbdk-lib/libc/targets/mos6502/nes/set_native_data.s

@@ -43,8 +43,8 @@ _set_sprite_native_data::
     rol
     asl *.ppu_addr_lo
     rol
-    bit *.crt0_forced_blanking
-    bpl .copy_tiles_indirect
+    bit *__oam_valid_display_on
+    bvc .copy_tiles_indirect
     ; Direct write
     sta PPUADDR
     lda *.ppu_addr_lo

gbdk-lib/libc/targets/mos6502/nes/vram_transfer_buffer.s

@@ -69,8 +69,8 @@ __vram_transfer_buffer_pos_old::        .ds 1
 ;
 .ppu_stripe_begin_horizontal::
     clc
-    bit *.crt0_forced_blanking
-    bpl .ppu_stripe_begin_indirect
+    bit *__oam_valid_display_on
+    bvc .ppu_stripe_begin_indirect
     ; Direct write
     stx PPUADDR
     sta PPUADDR
@@ -86,8 +86,8 @@ __vram_transfer_buffer_pos_old::        .ds 1
 ;
 .ppu_stripe_begin_vertical::
     sec
-    bit *.crt0_forced_blanking
-    bpl .ppu_stripe_begin_indirect
+    bit *__oam_valid_display_on
+    bvc .ppu_stripe_begin_indirect
     ; Direct write
     stx PPUADDR
     sta PPUADDR
@@ -142,8 +142,8 @@ __vram_transfer_buffer_pos_old::        .ds 1
 ; End a stripe (be it direct or via transfer buffer)
 ;
 .ppu_stripe_end::
-    bit *.crt0_forced_blanking
-    bpl 1$
+    bit *__oam_valid_display_on
+    bvc 1$
     ; For direct writes there's nothing more to do
     rts
 1$:
@@ -175,8 +175,8 @@ __vram_transfer_buffer_pos_old::        .ds 1
 ; Writes a byte of an in-progress horizontal / vertical stripe (be it direct or via transfer buffer)
 ;
 .ppu_stripe_write_byte::
-    bit *.crt0_forced_blanking
-    bpl 1$
+    bit *__oam_valid_display_on
+    bvc 1$
     sta PPUDATA     ; Direct write
     rts
 1$:
@@ -208,8 +208,8 @@ _set_vram_byte::
 .ppu_stripe_append::
     .define ppu_addr ".tmp"
     ;
-    bit *.crt0_forced_blanking
-    bpl .ppu_stripe_append_indirect
+    bit *__oam_valid_display_on
+    bvc .ppu_stripe_append_indirect
     ; Direct write
     stx PPUADDR
     sta PPUADDR