Writing a ROM
Sample code
This code originally appeared in the July 1987 issue of Computing With The Amstrad CPC. Please be aware that I scanned and OCRed the code from a photocopy of the magazine, and this may have led to some errors. I have proof-read this, but I'm not perfect. I've edited the code to suit the changes I made to the accompanying article but have not tested it (yay! quality assurance!) - DC.
;-------------------------------------- ;Amstrad ROM ; ;By R.A.Waddilove ;-------------------------------------- WRITE "CWTAROM.BIN" NOLIST LIMIT &FFFF ORG &C000 ;-------------------------------------- ;Reserves 32 bytes for workspace: ;0-17 used by DOUBLE ;-------------------------------------- ;ROM Prefix ;-------------------------------------- DEFB 1 ;ROM type ... background DEFB 1 ;mark 1 DEFB 1 ;version 1 DEFB 1 ;modification 1 ;-------------------------------------- ;Command table ;-------------------------------------- DEFW name_table JP initialise_ROM ;power-up/reset entry JP help ;help screen JP double ;double height printing ;-------------------------------------- ;Name table ;-------------------------------------- .name_table DEFB "CWTA RO","M"+&80 ;bad name DEFB 'HEL',"P'+&80 ;HELP DEFB "DOUBL","E"+&80 ;DOUBLE,@a$ DEFB 0 ;--------------------------------------- ;Power-up/Reset intialisation ;AF,BC corrupted ;--------------------------------------- .initialise_ROM PUSH DE:PUSH HL ;save DE/HL CALL print_string; print power-up message DEFB 13,10 DEFB "Computing with the Amstrad ROM" DEFB 13,10,10 DEFB 0 POP HL:POP DE ;restore HL/DE AND A LD BC,32:SBC HL,BC ;grab 32 bytes from top of memory SCF RET ;-------------------------------------- ;General string print subroutine ;AF,HL corrupted ;-------------------------------------- .print_string POP HL ;get string address . sp_loop LD A,(HL):CALL &BB5A ;print character INC HL OR A ;done? JR NZ,sp_loop JP (HL) ;--------------------------------------- ; HELP ; Print syntax and function of commands ; AF,HL corrupted ;-------------------------------------- .help CALL print_string DEFB "Computing with the Amstrad ROM",10, 13 DEFB "HELP - list all commands", 10, 13 DEFB "DOUBLE,@a$ - double height print", l0, 13 DEFB "Names can be prefixed with CWTA", l0, 13 DEFB 7,0 RET ;-------------------------------------- ;DOUBLE ; Double height print ; AF,BC,DE,HI corrupted ;-------------------------------------- .double DEC A JP NZ,help ;one parameter? LD L,(IX+0) LD H,(IX+1) ;HL points to string descriptor LD A,(HL) ;A=length AND A:RETZ ;abort if 0 LD (IY+17),A ;workspace+17=length INC HL: LD E,(HL) INC HL: LD D,(HL) ;DE=address .loop1 PUSH DE LD A,(DE) ;get character CALL &BBA5 ;HL=data address CALL &B906: PUSH AF ;enable lower ROM PUSH IY: POP DE ;DE=workspace LD B,8 ;copy data to workspace .loop2 LD A,(HL) LD (DE),A:INC DE LD (DE),A:INC DE INC HL DJNZ loop2 POP AF CALL &B90C ;restore lower ROM state LD A,255 ;define top half of character PUSH IY POP HL CALL &BBA8 LD A,254 ;define bottom half of character PUSH IY POP HL LD DE,8:ADD HL,DE CALL &BBA8 CALL print_string ;print big character DEFB 255,8,10,254,11,0 POP DE:INC DE ;next character DEC (IY+17):JR NZ,loop1 ;all done? RET list END **** END *
