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@pjgpetecodes pjgpetecodes/sprite1_2.asm
Last active Nov 10, 2017

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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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commented Oct 12, 2017

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.

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