;
; Ullrich von Bassewitz, 02.04.1999
;
; unsigned char getcpu (void);
;

        .export         _getcpu

; ---------------------------------------------------------------------------
; Subroutine to detect an 816. Returns
;
;   - carry clear and 0 in A for a NMOS 6502 CPU
;   - carry set and 1 in A for a 65C02
;   - carry set and 2 in A for a 65816
;   - carry set and 3 in A for a 4510
;   - carry set and 4 in A for a 65SC02
;   - carry set and 5 in A for a 65CE02
;   - carry set and 6 in A for a HuC6280
;   - carry clear and 7 in A for a 2a03/2a07
;
; This function uses a $1A opcode which is a INA on the 816 and ignored
; (interpreted as a NOP) on a NMOS 6502. There are several CMOS versions
; of the 6502, but all of them interpret unknown opcodes as NOP so this is
; just what we want.

.p816                           ; Enable 65816 instructions

_getcpu:
        lda     #0
        inc     a               ; .byte $1A ; nop on nmos, inc on every cmos
        cmp     #1
        bcc     @L8

; This is at least a 65C02, check for a 65CE02/4510

        .byte   $42,$EA         ; neg on 65CE02/4510, nop #$EA on 65C02, wdm $EA on 65816
        cmp     #1
        beq     @L6

; This is at least a 65CE02, check for 4510

        lda     #5              ; CPU_65CE02 constant
        .byte   $5C             ; map on 4510, aug on 65CE02 (acts like 4 byte nop)
        lda     #3              ; CPU_4510 constant
        nop
        bne     @L9

; 6502 type of cpu, check for a 2a03/2a07
@L8:
        sed                     ; set decimal mode, no decimal mode on the 2a03/2a07
        lda     #9
        clc
        adc     #1              ; $01+$09 = $10 on 6502, $01+$09 = $0A on 2a03/2a07
        cld
        cmp     #$0a
        beq     @L5
        lda     #0              ; CPU_6502 constant
        beq     @L9
@L5:
        lda     #7              ; CPU_2A0x constant
        bne     @L9

; 65C02 cpu type, check for HuC6280
@L4:    ldx     #6              ; CPU_HUC6280 constant
        .byte   $22,$EA         ; sax nop on HuC6280 (A=$06, X=$01), nop #$EA on 65C02 (A=$01, X=$06)
        bne     @L9

; Check for 65816/65802
@L6:    xba                     ; .byte $EB, put $01 in B accu (nop on 65C02/65SC02)
        dec     a               ; .byte $3A, A=$00
        xba                     ; .byte $EB, A=$01 if 65816/65802 and A=$00 if 65C02/65SC02
        inc     a               ; .byte $1A, A=$02 if 65816/65802 and A=$01 if 65C02/65SC02
        cmp     #2
        beq     @L9

; check for 65SC02

        ldy     $F7
        ldx     #0
        stx     $F7
        .byte   $F7,$F7         ; nop nop on 65SC02, smb7 $F7 on 65C02
        ldx     $F7
        sty     $F7
        cpx     #$00
        bne     @L4
        lda     #4              ; CPU_65SC02 constant

@L9:    ldx     #0              ; Load high byte of word
        rts