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1.1 Restarts

One useful feature of the 8080 was the ability to call a handful of addresses in low memory with a single-byte instuction, as opposed to the usual three bytes needed for CALL and other branch instructions. These addresses were known as the "Restart" addresses, and memory-conscious programmers would always put their most-commonly called functions here, thus saving two bytes on every call elsewhere in the program. There are 7 restart addresses, spaced at 8 byte intervals from 0000 to 0030 inclusive. (NB: There is support for an eighth restart function, RST 7, but Basic makes no use of it).


Start (RST 0)

Once the loader had finished loading Basic into memory from paper tape it would jump to address 0000, the very start of the program. Not much needs to be done here - just disable interrupts and jump up to the Init section in upper memory. Notice that the jump address here is coloured red, indicating the code is modified by code elsewhere. In this case, the jump address is changed to point to Ready once Init has run successfully. (fixme: not yet it isnt).

0000 F3 Start DI
0001 C3210D JMP Init

Fixme: What are these two addresses for?

0004 9004 DW 0490
0006 F907 DW 07F9

SyntaxCheck (RST 1)

Here is a truly beautiful piece of code, it's Golden Weasel richly deserved. It's used at run-time to check syntax in a very cool way : the byte immediately following an RST 1 instruction is not the following instruction, but the keyword or operator ID that's expected to appear in the program at that point. If the keyword or operator is not present, then it Syntax Errors out, but if it is present then the return address is fixed-up - ie advanced one byte - and the function falls into NextChar so the caller has even less work to do. I honestly doubt syntax checks could be done more efficiently than this. Sheer bloody genius.

0008 7E SyntaxCheck MOV A,M A=Byte of BASIC program.
0009 E3 XTHL HL=return address.
000A BE CMP M Compare to byte expected.
000B 23 INX H Return address++;
000C E3 XTHL
000D C2D001 JNZ SyntaxError Error if not what was expected.

NextChar (RST 2)

Return next character of input from the buffer at HL, skipping over space characters. The Carry flag is set if the returned character is not alphanumeric, also the zero flag is set if a null character has been reached.

0010 23 NextChar INX H
0011 7E MOV A,M
0012 FE3A CPI 3A
0014 D0 RNC
0015 C35E04 JMP NextChar_tail

OutChar (RST 3)

Prints a character to the terminal.

0018 F5 OutChar PUSH PSW
0019 3A2700 LDA TERMINAL_X
001C C36E03 JMP OutChar_tail
001F 00 NOP

CompareHLDE (RST 4)

Compares HL and DE with same logical results (C and Z flags) as for standard eight-bit compares.

0020 7C CompareHLDE MOV A,H
0021 92 SUB D
0022 C0 RNZ
0023 7D MOV A,L
0024 93 SUB E
0025 C9 RET


Variables controlling the current X and Y positions of terminal output.

0026 01 TERMINAL_Y DB 01
0027 00 TERMINAL_X DB 00

FTestSign (RST 5)

Tests the state of FACCUM. This part returns with A=0 and zero set if FACCUM==0, the tail of the function sets the sign flag and A accordingly (0xFF is negative, 0x01 if positive) before returning.

0028 3A7201 FTestSign LDA FACCUM+3
002B B7 ORA A
002C C2DA09 JNZ FTestSign_tail
002F C9 RET

PushNextWord (RST 6)

Effectively PUSH (HL). First we write the return address to the JMP instruction at the end of the function; then we read the word at (HL) into BC and push it onto the stack; lastly jumping to the return address.

0030 E3 PushNextWord XTHL
0031 223B00 SHLD 003B
0034 E1 POP H
0035 4E MOV C,M
0036 23 INX H
0037 46 MOV B,M
0038 23 INX H
0039 C5 PUSH B
003A C33A00 JMP 003A


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