SPACE key on PS/2 keyboard return scancode of 0x29  (41 in decimal). In Zeddies the SPACE is at line 0 (KB0) of  row 7 (A15)

Then the 41th positon in vector   PS2Keymap[ ] contains the value 0x07

Backspace has a PS/2 scancode of 0x66 (102) and is a good example of the use of the modifier. On the Zeddies the corresponding key is SHIFT + '0' (ROBOUT). Key 0 itself is at line 0 of row 4. Then the 102th entry in vector  PS2Keymap[ ]  has the value of 04 for the '0' key plus  128 that is the 7th bit for the encoded value.

Arrow keys are extended  PS/2 codes but are also mapped as SHIFT plus the values for keys 5 to 8.

...
if (EXT==true) { // extended set
    EXT=false;
    switch (code) {
        case _PS2_UP:   	m=0x80 | _7; break; // Caps Shift bit + map code for ZX key 7
        case _PS2_DOWN: 	m=0x80 | _6; break;
        case _PS2_LEFT: 	m=0x80 | _5; break;
        case _PS2_RIGHT:	m=0x80 | _8; break;
        case _PS2_KPENT:	m=_ENT     ; break; // Enter key
        case _PS2_LCONTROL:	m=_SYMB    ; break; // Enter key
	...
...

The function Update_matrix( ) uses this value  to SET or RESET a bit on the keyboard matrix depending upon the reception of a 'break' code previously.

void Update_matrix( char m){
uint8_t line= m & 0x07;
uint8_t row=  (m>>3) & 0x07;
	
if (BRK==true) { 
    BRK=false;
    Keymap[line]|=(1<<row);          //  set bits to break
    if (m & 0x80) Keymap[0]|=(1<<0); // if bit 7 is set then set Caps Shift bit at row=0, line=0
    if (m & 0x40) Keymap[7]|=(1<<1); // if bit 6 is set then set Symbol Shift bit at row=1, line=7			
} else {                              
    Keymap[line]&=~(1<<row);          // reset bits to make
    if (m & 0x80) Keymap[0]&=~(1<<0); // if bit 7 is set then reset Caps Shift bit at row=0, line=0
    if (m & 0x40) Keymap[7]&=~(1<<1); // if bit 6 is set then reset Symbol Shift bit at row=1, line=7	
}