Filter
Top Products
318 Rabbit 3000 Microprocessor
C.1 System/User Mode Opcodes
Seven new opcodes have been added to support the System/User mode, and are listed in
Table C-2. All but IDET are placed in previously empty opcode table assignments. IDET
shares the value of LD E,E in the opcode table, and will perform that operation when the
System/User mode is disabled, or when it is enabled and in the System mode. In addition,
if the ALTD prefix appears before the opcode, LD E’,E is always executed instead.
The processor keeps a one-byte stack (called the SU register) that is analogous to the IP
register that keeps track of the interrupt priority. Every time SETUSR is executed (to enter
the User mode), or an interrupt occurs, or SYSCALL or RST is executed (to enter the Sys-
tem mode), the current mode is pushed onto the SU register. When a SURES is executed,
the previous mode is popped off the SU register.
The effects of each opcode are:
• The SETUSR opcode puts the processor into the User mode by pushing the correct value
into the SU register.
• PUSH SU and POP SU push and pop the single-byte SU register on/off the SP stack.
• SURES pops the current processor mode off the SU register, returning it to the previous
mode.
• IDET causes an interrupt if executed in the User mode, and does nothing in the System
mode.
• RDMODE returns the current mode in the carry flag (0 for System mode, 1 for User mode).
• SYSCALL is essentially a new RST opcode, and was added to allow User mode access to
the System mode without using one of the existing RST opcodes. It will put the processor
into the System mode and execute code in the corresponding interrupt-vector table entry.
Table C-2. New System/User Mode Opcodes
Instruction Bytes clk A I S Z V C Operation
Priv
?
SETUSR
2 4 -----SU = {SU[5:0], 0x01} Yes
PUSH SU
2 9 -----(SP-1) = SU; SP = SP - 1 Yes
POP SU
2 7 -----SU = (SP); SP = SP + 1 Yes
SURES
2 4 -----SU = {SU[1:0], SU[7:2]} Yes
IDET
1 2 -----
Performs
LD E,E, but if
(EDMF && SU[0]) then the System
Violation interrupt flag is set; if ALTD
appears before it always does
LD E’,E
No
RDMODE
2 4 ----*CF = SU[0] Yes
SYSCALL
2 10 -----
SP = SP - 2; PC = {R,v} where
v = SYSCALL offset
No