ST6 and ST7

ST6 architecture

The ST6 has 64 bytes of RAM and 4096 bytes of program ROM. Larger amounts are accessed by bank-switching the low 2K section of the ROM.

The RAM address space is actually 256 bytes, divided as follows:

• 0–63: Not implemented
• 64–127: Bank-switchable window into program ROM and data EPROM.
• 128–191: General-purpose RAM
• 192–255: Peripheral control registers (GPIO ports, timers, etc.) The accumulator is mapped at address 255, but is more commonly addressed implicitly.

Not mapped into the address space is a 12-bit program counter and an associated hardware stack (four or six levels deep, depending on model). There are only two status bits (carry and zero), and they are banked based on processor mode, with separate status bits for normal, interrupt and non-maskable interrupt operation.

The first four general-purpose RAM locations are also known as the X, Y, V and W registers, and some instructions can access them using special short addressing modes. The X and Y registers serve as index registers, and can use indirect addressing modes (X) and (Y).

The instruction set consists of one byte of opcode, followed by up to two one-byte operands. The instruction set can be summarized as follows:

Carry note†: ^ a b Confusingly, different models of the ST6 family use different conventions for the value of the carry bit after a subtraction. ST60 processors use the "carry" convention, which clears the bit if the subtract underflows, while the ST62 and ST63 processors use the "borrow" convention, which sets the bit in that case.

ST7 architecture

The ST7 has six registers: the accumulator, X and Y index registers, stack pointer, program counter, and condition code register. Also, double-indirect addressing allows the zero page of RAM to serve as additional registers. An unusual but useful feature is that an interrupt pushes four of these registers on the stack (A and X as well as the usual PC and CC), and interrupt return restores them.

ALU instructions fall into two categories, two-operand and one-operand.

Two-operand instructions use the accumulator as the first source. The addressing mode specifies the second source, which may be:

• 8-bit immediate
• 8-bit absolute address
• 16-bit absolute address
• Indexed (X)
• Indexed plus 8-bit offset (address8,X)
• Indexed plus 16-bit offset (address16,X) The destination is usually the accumulator, but a few instructions modify the second source. (Immediate operands are forbidden in such cases.)

One-operand instructions use the specified operand for both source and destination. The operand may be:

• The accumulator A
• The X register
• 8-bit absolute address
• Indexed (X)
• Indexed plus 8-bit offset (address8,X)

Register plus offset computes a full-width sum, so the 8-bit form may address memory up to 255+255 = 510.

In addition to the above, there are three prefix bytes which may be prepended to any instruction for which they make sense:

• PDY (0x90) changes all references to the X register to Y. This allows (Y), (address8,Y) and (address16,Y) addressing modes. This affects implicit operands as well, so the "load X" instruction becomes "load Y". A consequence of this is that load X can only use the X-relative addressing modes, and load Y can only use the Y-relative ones.
• PIX (0x92) adds an indirection step to the instruction. The 8- or 16-bit address following the opcode byte is replaced by an 8-bit address of a memory location which holds an 8- or 16-bit address (the latter in big-endian order). This may then be indexed by the X register as usual. This allows (address8), (address16), ([address8],X) and ([address8.w],X) addressing modes.
• PIY (0x91) combines the above effects. This allows the ([address8],Y) and ([address8.w],Y) addressing modes. (It may also be used with other modes as part of the "load Y" and "store Y" instructions.)

References

References

1. Datasheet: ST62T00C/T01C from 1998
2. "2006 EDN Microcontroller/Microprocessor directory, 8-bit microprocessors sorted by Instruction Set Architecture".
3. (October 2004). "ST6 Family Programming Manual". [[STMicroelectronics]].
4. (November 2005). "ST7 Family Programming Manual". [[STMicroelectronics]].