GMC-4 loader port layout

Key	Code	Port B: 43210	IC2 PB3:PB2	Row	IC1 PB4:PB1:PB0	Column	Crossing
0	00	00000	00	0,1,2,3,RST	000	0,4,8,C	0
1	01	00001	00	0,1,2,3,RST	001	1,5,9,D	1
2	02	00010	00	0,1,2,3,RST	010	2,6,A,E	2
3	03	00011	00	0,1,2,3,RST	011	3,7,B,F	3
4	04	00100	01	4,5,6,7,RUN	000	0,4,8,C	4
5	05	00101	01	4,5,6,7,RUN	001	1,5,9,D	5
6	06	00110	01	4,5,6,7,RUN	010	2,6,A,E	6
7	07	00111	01	4,5,6,7,RUN	011	3,7,B,F	7
8	08	01000	10	8,9,A,B,INC	000	0,4,8,C	8
9	09	01001	10	8,9,A,B,INC	001	1,5,9,D	9
A	0A	01010	10	8,9,A,B,INC	010	2,6,A,E	A
B	0B	01011	10	8,9,A,B,INC	011	3,7,B,F	B
C	0C	01100	11	C,D,E,F,ADR	000	0,4,8,C	C
D	0D	01101	11	C,D,E,F,ADR	001	1,5,9,D	D
E	0E	01110	11	C,D,E,F,ADR	010	2,6,A,E	E
F	0F	01111	11	C,D,E,F,ADR	011	3,7,B,F	F
RST	10	10000	00	0,1,2,3,RST	100	RST,RUN,INC,ADR	RST
RUN	14	10100	01	4,5,6,7,RUN	100	RST,RUN,INC,ADR	RUN
INC	18	11000	10	8,9,A,B,INC	100	RST,RUN,INC,ADR	INC
ADR	1C	11100	11	C,D,E,F,ADR	100	RST,RUN,INC,ADR	ADR

This is a simplified version of the original gmc.c.

#define GMC4_ADRSET 0x1C  /* 00011100 */
#define GMC4_INCR   0x18  /* 00011000 */
#define GMC4_RUN    0x14  /* 00010100 */
#define GMC4_RESET  0x10  /* 00010000 */

void gmc4wait(short waittime) { do {} while(waittime-- > 0); }

void gmc4init() {
    DDRB |= 0x1f;
    DDRB |= 1 << 6;
}

void gmc4_simulate_key(unsigned char key) {
    PORTB &= ~0x1f;
    PORTB |= 0x1f & key;
    
    PORTB &= ~(1 << 6);
    gmc4wait(8000);
    PORTB |= ~(1 << 6);
    gmc4wait(8000);
}

void gmc4sendkey(char key) {
    if (key >= '0' && key <= '9') {
        gmc4_simulate_key(key - '0');
    } else if(key >= 'A' && key <= 'F') {
        gmc4_simulate_key(key - 'A' + 10);
    } else if(key >= 'a' && key <= 'f') {
        gmc4_simulate_key(key - 'a' + 10);
    } else if(key == 'S' || key == 's') {
        gmc4_simulate_key(GMC4_ADRSET);
    } else if(key == 'I' || key == 'i') {
        gmc4_simulate_key(GMC4_INCR);
    } else if(key == 'G' || key == 'g') {
        gmc4_simulate_key(GMC4_RUN);
    } else if(key == 'R' || key == 'r') {
        gmc4_simulate_key(GMC4_RESET);
    }
}