THHN page 12:
The /SYS module is not accessed through the HIT (hashed name storage)
 sector but is found using the contents of the 'A' register, the lower 
bits of which contain the relative sector.
....
The top 3 bits of 'A' are used to differentiate between sub-functions. 
Sometimes other registers contain parameters for use by the 
module.

SYS*/SYS -> function & memory location: THHN p26
SYS*/SYS -> Track/Gran on disk: THHN p68

RST 0x28 in SYS0/SYS:

; No-Error exit
ld   a,0x23 ; 001 00011 3:1 ?SYS1/SYS? Tr.16/Gr.0
rst  0x28

;
ld   a,0x43 ; 010 00011 3:2 ?SYS1/SYS? Tr.16/Gr.0
rst  0x28

; Enter DOS and execute a command
ld   a,0x63 ; 011 00011 3:3 ?SYS1/SYS? Tr.16/Gr.0
rst  0x28

; Extract a filespec
ld   a,0x83 ; 100 00011 3:4 ?SYS1/SYS? Tr.16/Gr.0
rst  0x28

; Insert default name extension into filespec
ld   a,0xA3 ; 101 00011 3:5 ?SYS1/SYS? Tr.16/Gr.0
rst  0x28

; DOS-CALL Execute a DOS command and return
ld   a,0xC3 ; 110 00011 3:6 ?SYS1/SYS? Tr.16/Gr.0
rst  0x28

;
ld   a,0xE3 ; 111 00011 3:7 ?SYS1/SYS? Tr.16/Gr.0
ld   c,0x04
rst  0x28

; Open a FCB to a existing file
ld   a,0x24 ; 001 00100 4:1 ?SYS2/SYS? Tr.16/Gr.1
rst  0x28

; Open a FCB to new or existing disk file
ld   a,0x44 ; 010 00100 4:2 ?SYS2/SYS? Tr.16/Gr.1
rst  0x28

; Load a program file
ld   a,0xA4 ; 101 00100 4:5 ?SYS2/SYS? Tr.16/Gr.1
rst  0x28

; Load and commence execution of a program file
ld   a,0xC4 ; 110 00100 4:6 ?SYS2/SYS? Tr.16/Gr.1
rst  0x28

; Close a FCB
ld   a,0x25 ; 001 00101 5:1 ?SYS3/SYS? Tr.18/Gr.0
rst  0x28

; Kill FCB's associated file
ld   a,0x45 ; 010 00101 5:2 ?SYS3/SYS? Tr.18/Gr.0
rst  0x28

; Enqueue a user timer interrupt
ld   a,0x65 ; 011 00101 5:3 ?SYS3/SYS? Tr.18/Gr.0
rst  0x28

; Dequeue a user timer interrupt routine
ld   a,0x85 ; 100 00101 5:4 ?SYS3/SYS? Tr.18/Gr.0
rst  0x28

; 
ld a,0xA5 ; 101 00101 5:5 ?SYS3/SYS? Tr.18/Gr.0
rst  0x28

; Write the EOF value from the FCB to the directory
ld   a,0xC5 ; 110 00101 5:6 ?SYS3/SYS? Tr.18/Gr.0
rst  0x28

; DOS Error Exit
ld   a,0x26 ; 001 00110 6:1 - ?SYS4/SYS? Tr.18/Gr.1
rst  0x28

;
ld   a,0x46 ; 010 00110 6:4 - ?SYS4/SYS? Tr.18/Gr.1
rst  0x28

; Load and enter DEBUG
ld   a,0x27 ; 001 00111 7:1 - SYS5/SYS Tr.20/Gr.0
rst  0x28

; *name routine enqueue
ld   a,0x2B ; 001 01011 B:1
rst  0x28

; *name routine dequeue
ld   a,0x4B ; 010 01011 B:2
rst  0x28