Sam Coupe Sprite Tutorial - Part 2 - Stack Pointer Resolution
| ; | |
| ; ----------------------------------------------------------------------------------- | |
| ; | | | |
| ; | Program: SAM Coupe Sprites Tutorial 1 - Part 2 | | |
| ; | Filename: Sprites1_2.asm | | |
| ; | Version: 1.2 | | |
| ; | Date: 12/10/2017 | | |
| ; | Author: Pete Gallagher - PJG Creations Ltd | | |
| ; | | | |
| ; ----------------------------------------------------------------------------------- | |
| ; | |
| autoexec ; Automatically Execute the Program | |
| ORG 0x8000 ; Store the program in User Address Space | |
| DUMP 1,0 ; Store the Program in Page 1 (B) | |
| ; | |
| ; *********************************************************************************** | |
| ; * * | |
| ; * Equates * | |
| ; * * | |
| ; *********************************************************************************** | |
| ; | |
| LMPR: EQU 250 ; The Lower Memory Page Register Address | |
| HMPR: EQU 251 ; The Higher Memory Page Register Address | |
| VMPR: EQU 252 ; The Video Memory Page Register Address | |
| JCLSBL: EQU 334 ; The Clear Screen Routine... | |
| ; ... Set A to 0 to clear the whole screen) (0x14E) | |
| JREADKEY: EQU 361 ; The Keyboard Read Routine... | |
| ; ... Returns with Z Flag set if no key pressed... | |
| ; ... Otherwise A = Keycode for key pressed | |
| ; | |
| ; *********************************************************************************** | |
| ; * * | |
| ; * Application Start * | |
| ; * * | |
| ; *********************************************************************************** | |
| ; | |
| DI ; Disable Interupts | |
| ; | |
| ; Becuase the Stack Pointer lives in Page B, but we're paging the Screen into Pages A and B, | |
| ; we need to make sure we save the current Stack Pointer location, and create our own location. | |
| ; We rewrite our program here effectively, by storing it's original location in a holding position. | |
| ; | |
| LD (System_SP+1),SP; Get the Current Stack Pointer Location and Overwrite... | |
| ; ... the holding value below. | |
| ; | |
| ; Get the Current Screen Page and set the LMPR register up so we can write to the screen | |
| ; | |
| IN A,(VMPR) ; Get the VMPR Register, which holds info about... | |
| ; ... which Page the Screen is Paged Into | |
| AND 0b00011110 ; We're only interested in the Screen Page... | |
| ; ... so mask off everything but the Page Bits... | |
| ; ... The result will be stored in the A Register | |
| OR 0b00100000 ; Set Bit 5 which, when loaded into the LMPR register... | |
| ; ... will Set the RAM in Page 0 | |
| LD (Scr_Page+1),A ; Overwrite the holding value for the Screen Page location... | |
| ; ... now that we know it | |
| ; | |
| CALL Clear_Screen ; Clear the Screen | |
| ; | |
| LD SP,0xC000 ; Set the Stack Pointer to a location of our choosing | |
| ; | |
| LD A,(Scr_Page+1) ; Get the current Screen Location which we stored earlier | |
| ; | |
| OUT (LMPR),A ; | |
| ; | |
| ; All of the above shifting around of the Stack Pointer, allows us to Call routines... | |
| ; ... Which of course will store their return location on the Stack Pointer... | |
| ; ... This would've overwritten some of the Screen Page if left unreseolved... | |
| ; | |
| CALL Wait_For_Key ; Wait for a keypress | |
| ; | |
| ; Print a sprite in the top left corner | |
| ; | |
| LD HL,0x0000 ; Point to the Top Left corner of our screen | |
| LD (HL),0xFF ; Set the colour of pixels 0,0 and 0,1 to White | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0080 ; | |
| LD (HL),0x44 ; Set the colour of pixels 1,0 and 1,1 to Green | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0001 ; | |
| LD (HL),0xFF ; Set the colour of pixels 0,2 and 0,3 to White | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0081 ; | |
| LD (HL),0x44 ; Set the colour of pixels 1,2 and 1,3 to Green | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0100 ; | |
| LD (HL),0xFF ; Set the colour of pixels 1,0 and 1,1 to White | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0180 ; | |
| LD (HL),0x44 ; Set the colour of pixels 2,0 and 2,1 to Green | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0101 ; | |
| LD (HL),0xFF ; Set the colour of pixels 1,2 and 1,3 to White | |
| CALL Wait_For_Key ; Wait for a keypress | |
| LD HL,0x0181 ; | |
| LD (HL),0x44 ; Set the colour of pixels 2,2 and 2,3 to Green | |
| CALL Wait_For_Key ; Wait for a keypress | |
| ; | |
| ; We must remember to switch BASIC back into Page 0 | |
| ; | |
| LD A,0x1F ; Set ROM0 back into Page A... | |
| OUT (LMPR), A ; ... So that when we try to return to BASIC it all still works! | |
| ; | |
| ; Return the System Stack Pointer to it's original location in Bank B somewhere... | |
| ; | |
| System_SP: LD SP,0 ; Place holder for the System Stack Location... | |
| ; This will be overwritten above once we know it's location | |
| EI ; Enable Interupts | |
| ; | |
| ; Return back to BASIC | |
| ; | |
| RET ; | |
| ; | |
| ; *********************************************************************************** | |
| ; * * | |
| ; * Clear the Screen * | |
| ; * * | |
| ; *********************************************************************************** | |
| ; | |
| Clear_Screen: XOR A ; Clear the A register, so when passed into clear screen... | |
| ; ...it clears the whole screen | |
| CALL JCLSBL ; Clear the Screen | |
| DI ; Disable Interupts | |
| RET ; | |
| ; | |
| ; *********************************************************************************** | |
| ; * * | |
| ; * Wait for a KeyPres * | |
| ; * * | |
| ; *********************************************************************************** | |
| ; | |
| Wait_For_Key: LD A,0x1F ; Set for ROM in Page A | |
| OUT (LMPR),A ; | |
| ; | |
| Key_Loop: CALL JREADKEY ; Read the Keyboard (Zero if no key) | |
| JR Z,Key_Loop ; If no key pressed, then loop | |
| ; | |
| No_Key_Loop: CALL JREADKEY ; Read the Keyboard (Zero if no key) | |
| JR NZ,No_Key_Loop ; If key still pressed, then loop | |
| ; | |
| DI ; Disable Interuprts | |
| ; | |
| Scr_Page: LD A,0 ; Place holder for the Screen Page... | |
| ; ...The 0 will be overwritten above once we know where it is... | |
| OUT (LMPR),A ; Page the Screen into the relevant Page | |
| RET ; |
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This is an updated version of https://gist.github.com/PGallagher69/5f30d3e27574e5847027e39c7c5d878d, which caused the Screen to display coloured blocks.
This was because the Stack Pointer was overwriting the Paged in Screen Memory.