Saving and loading of machine code in BBC BASIC
- Gunther Strube
- cambridgez88
Owned by Gunther Strube
The simplest method of loading a machine code is having it stored in DATA statements as byte opcode sequenses. However, this emplies a lot of work, converting an assembled code into DATA statements (you could make a BASIC program that generates a simple text file that later could could be typed in by CLI). Another way is simply to have the in-line assembler source included together with the rest of the BBC BASIC program. However, large assembler source tends to use much memory, especially if it is well-commented. It is usually sufficient with small assembler routines.
When you use pre-compiled code ready to execute, you face another obstacle; you must always place the code at the same location. Using DIM will probably not return the same address of allocated memory, so it is needed to move down HIMEM to ensure a static position for your code. If you have the source in-line then it is no problem, since you always can re-compile for a specifically allocated memory area.
The two BASIC procedures below implements a simple interface to both load and save machine code in binary files. Both procedures use a small machine code routine to save/load the code in BASIC's memory. To ensure compact BASIC procedures, the machine code have been put into DATA statements, placed inside the procedures. The mnemonic assembler source are found below the procedures. Both procedures locate the machine code routine at the bottom of the stack, beginning at $1800. This area is not touched by BBC BASIC and is therefore safe. However, when BBC BASIC has been pre-empted the bottom of the stack might have changed. Please note that both routines null-terminate the filename strings.
65515 DEF PROC_lbytes(f$,addr%)Â 65516 LOCAL b%,i% 65517 f$=f$+CHR$0Â 65518 RESTORE 65520: FOR i%=0 TO 66: READ b%: ?(&1800+i%)=b%: NEXT i%Â 65519 CALL &1800,f$,addr%Â 65520 DATA &21,0,0,&39,&ED,&7B,&FE,&1F,&E5,&CD,&0F,&18,&E1,&F9,&C9Â 65521 DATA &DD,&E5,&FD,&E1,&FD,&6E,2,&FD,&66,3,&23,&23,&5E,&23,&56,&62Â 65522 DATA &6B,1,2,0,&3E,1,&E7,9,&60,&38,&15,&FD,&6E,5,&FD,&66,6Â 65523 DATA &5E,&23,&56,&21,0,0,1,&FF,&FF,&E7,&45,&E7,9,&62,&C9,&E7,9,&4A,&C9 65524 ENDPROC 65525 DEF PROC_sbytes(f$,addr%,length%)Â 65526 LOCAL b%,i% 65527 f$=f$+CHR$0Â 65528 RESTORE 65530: FOR i%=0 TO 73: READ b%: ?(&1800+i%)=b%: NEXT i%Â 65529 CALL &1800,f$,addr%,length%Â 65530 DATA &21,0,0,&39,&ED,&7B,&FE,&1F,&E5,&CD,&0F,&18,&E1,&F9,&C9Â 65531 DATA &DD,&E5,&FD,&E1,&FD,&6E,2,&FD,&66,3,&23,&23,&5E,&23,&56,&62Â 65532 DATA &6B,1,2,0,&3E,2,&E7,9,&60,&38,&1C,&FD,&6E,5,&FD,&66,6Â 65533 DATA &5E,&23,&56,&FD,&6E,8,&FD,&66,9,&4E,&23,&46,&EB,&11,0,0Â 65534 DATA &E7,&45,&E7,9,&62,&C9,&E7,9,&4A,&C9Â 65535 ENDPROC
Source code of sbytes function
MODULE Lbytes  include "fileio.def"           ; standard file I/O definitions include "error.def"            ; error code definitions ORG $1800                          ; subroutine resided at $1800 ;**************************************************************************** ; BBC BASIC 'load bytes' machine code routine. The routine always loads the ; complete file (length), ; which is not specified in the CALL parameter list. ; ;          1st parameter is the string pointer ;          2nd parameter is the start address integer ; ; IN: IX = pointer to BASIC parameter block: ;          (IX+0)     number of parameters:              2 ;          (IX+1)     type of 1st parameter:             129 (mov. string) ;          (IX+2,3)   address of movable string:         xx ;          (IX+4)     type of 2nd parameter:             4 (32bit integer) ;          (IX+5,6)   address of 2nd parameter:          xx ;          The movable string parameter block: ;          (xx+0)     current length ;          (xx+1)     max. length ;          (xx+2,3)   start address of string ;**************************************************************************** .InitLbytes ld  hl,0            add hl,sp             ; get current BASIC stack pointer            ld  sp,($1FFE)        ; install safe stack pointer            push hl                ; preserve BASIC stack pointer            call Lbytes            pop hl            ld  sp,hl             ; restore BASIC stack            ret                    ; return to BASIC interpreter .Lbytes    push ix            pop iy                ; iy points at CALL parameter block            ld  l,(iy+2)            ld  h,(iy+3)          ; get pointer to moveable string par. block            inc hl            inc hl                ; point at string start address pointer            ld  e,(hl)            inc hl            ld  d,(hl)            ld  h,d               ; HL & DE = pointer to start of filename            ld  l,e            ld  bc,2              ; local pointer, C = 2 byte scratch buffer            ld  a, OP_IN            oz GN_Opf            jr  c, file_err       ; Ups, file couldn't be opened            ld  l,(iy+5)          ; IX = file handle            ld  h,(iy+6)            ld  e,(hl)            inc hl            ld  d,(hl)            ; start address to load machine code            ld  hl,0            ld  bc, $FFFF         ; max. file length (probably much snaller!)            oz OS_Mv          ; load file image at (DE) onwards...            oz GN_Cl          ; close file            ret .file_err  oz GN_Err        ; display error box...            ret
Source code of sbytes function
MODULE Sbytes  include "fileio.def"           ; standard file I/O definitions include "error.def"            ; error code definitions ORG $1800                          ; subroutine resided at $1800 ;**************************************************************************** ; BBC BASIC 'save bytes' machine code routine. ;          1st parameter is the string pointer ;          2nd parameter is the start address integer ;          3rd parameter is length of memory block ; ; IN: IX = pointer to BASIC parameter block: ;          (IX+0)     number of parameters:              2 ;          (IX+1)     type of 1st parameter:             129 (mov. string) ;          (IX+2,3)   address of movable string:         xx ;          (IX+4)     type of 2nd parameter:             4 (32bit integer) ;          (IX+5,6)   address of 2nd parameter:          xx ;          (IX+7)     type of 3rd parameter:             4 (32bit integer) ;          (IX+8,9)   address of 3rd parameter:          xx ; ;          The movable string parameter block: ;          (xx+0)     current length ;          (xx+1)     max. length ;          (xx+2,3)   start address of string ; ;**************************************************************************** .InitSbytes ld  hl,0            add hl,sp             ; get current BASIC stack pointer            ld  sp,($1FFE)        ; install safe stack pointer            push hl                ; preserve BASIC stack pointer            call Sbytes            pop hl            ld  sp,hl             ; restore BASIC stack            ret                    ; return to BASIC interpreter .Sbytes    push ix            pop iy                ; iy points at CALL parameter block            ld  l,(iy+2)            ld  h,(iy+3)          ; get pointer to moveable string par. block            inc hl            inc hl                ; point at string start address pointer            ld  e,(hl)            inc hl            ld  d,(hl)            ld  h,d               ; HL & DE = pointer to start of filename            ld  l,e            ld  bc,2              ; local pointer, C = 2 byte scratch buffer            ld  a, OP_OUT            oz GN_Opf            jr  c, file_err       ; Ups, file couldn't be created            ld  l,(iy+5)          ; IX = file handle            ld  h,(iy+6)          ; HL = pointer to pointer to start address            ld  e,(hl)            inc hl            ld  d,(hl)            ; DE = start addr. of memory block to save            ld  l,(iy+8)            ld  h,(iy+9)          ; HL = pointer to pointer to length            ld  c,(hl)            inc hl            ld  b,(hl)            ; BC = length of memory block to save            ex  de,hl            ld  de,0            oz OS_Mv          ; save memory block from (HL) onwards...            oz GN_Cl          ; close file            ret .file_err  oz GN_Err         ; display error box...            ret